The fair value and gross unrealized losses of available-for-sale securities, aggregated by investment category, were as follows:
| | | | | | | | | | | | | | | | | | | | | | | | | | June 30, 2013 | | Less than 12 months | | 12 months or longer | | Total | | Fair Value | | Gross Unrealized Losses | | Fair Value | | Gross Unrealized Losses | | Fair Value | | Gross Unrealized Losses | Available-for-sale securities | | | | | | | | | | | | Asset-backed securities | $ | 300.6 |
| | $ | (4.1 | ) | | $ | 77.6 |
| | $ | (0.5 | ) | | $ | 378.2 |
| | $ | (4.6 | ) | Commercial-mortgage-backed securities | 28.7 |
| | (0.3 | ) | | — |
| | (0.4 | ) | | 28.7 |
| | (0.7 | ) | Corporates | 3,133.5 |
| | (142.4 | ) | | 396.0 |
| | (21.1 | ) | | 3,529.5 |
| | (163.5 | ) | Hybrids | 75.4 |
| | (2.5 | ) | | 40.7 |
| | (0.6 | ) | | 116.1 |
| | (3.1 | ) | Equities | 90.9 |
| | (3.2 | ) | | 33.9 |
| | (1.4 | ) | | 124.8 |
| | (4.6 | ) | Municipals | 350.9 |
| | (21.4 | ) | | 101.4 |
| | (6.0 | ) | | 452.3 |
| | (27.4 | ) | Agency residential mortgage-backed securities | 2.5 |
| | — |
| | 6.3 |
| | (0.4 | ) | | 8.8 |
| | (0.4 | ) | Non-agency residential mortgage-backed securities | 255.5 |
| | (8.0 | ) | | 69.2 |
| | (1.3 | ) | | 324.7 |
| | (9.3 | ) | U.S. Government | 198.9 |
| | (3.9 | ) | | — |
| | — |
| | 198.9 |
| | (3.9 | ) | Total available-for-sale securities | $ | 4,436.9 |
| | $ | (185.8 | ) | | $ | 725.1 |
| | $ | (31.7 | ) | | $ | 5,162.0 |
| | $ | (217.5 | ) | Total number of available-for-sale securities in an unrealized loss position | | | 537 |
| | | | 106 |
| | | | 643 |
|
| | | | | | | | | | | | | | | | | | | | | | | | | | September 30, 2012 | | Less than 12 months | | 12 months or longer | | Total | | Fair Value | | Gross Unrealized Losses | | Fair Value | | Gross Unrealized Losses | | Fair Value | | Gross Unrealized Losses | Available-for-sale securities | | | | | | | | | | | | Asset-backed securities | $ | 169.8 |
| | $ | (1.0 | ) | | $ | 7.5 |
| | $ | (0.6 | ) | | $ | 177.3 |
| | $ | (1.6 | ) | Commercial-mortgage-backed securities | 0.8 |
| | (0.8 | ) | | 10.7 |
| | (1.6 | ) | | 11.5 |
| | (2.4 | ) | Corporates | 411.3 |
| | (8.1 | ) | | 45.5 |
| | (1.9 | ) | | 456.8 |
| | (10.0 | ) | Equities | — |
| | — |
| | 44.5 |
| | (1.3 | ) | | 44.5 |
| | (1.3 | ) | Hybrids | 13.4 |
| | (0.4 | ) | | 107.7 |
| | (9.2 | ) | | 121.1 |
| | (9.6 | ) | Municipals | 71.1 |
| | (1.1 | ) | | — |
| | — |
| | 71.1 |
| | (1.1 | ) | Agency residential mortgage-backed securities | 1.8 |
| | (0.2 | ) | | 6.1 |
| | (0.1 | ) | | 7.9 |
| | (0.3 | ) | Non-agency residential mortgage-backed securities | 12.9 |
| | (0.3 | ) | | 101.8 |
| | (4.0 | ) | | 114.7 |
| | (4.3 | ) | Total available-for-sale securities | $ | 681.1 |
| | $ | (11.9 | ) | | $ | 323.8 |
| | $ | (18.7 | ) | | $ | 1,004.9 |
| | $ | (30.6 | ) | Total number of available-for-sale securities in an unrealized loss position | | | 100 |
| | | | 56 |
| | | | 156 |
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At June 30, 2013 and September 30, 2012, securities in an unrealized loss position were primarily concentrated in investment grade corporate debt instruments, residential mortgage-backed securities and municipals. Total unrealized losses were $217.5 and $30.6 at June 30, 2013 and September 30, 2012, respectively. The increase in the unrealized loss position is largely due to the increase in Treasury yields over the reporting period. 10 year US Treasury yields increased from 1.63% at September 30, 2012 to 2.48% at June 30, 2013. As a result, corporate debt holdings generally declined in value during this period. The increase in Treasury yields during the June fiscal
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