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Investment Securities (Other Long-Term Investments) (Details) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
--- | --- | --- | ---
Investment securities |  |  |
Cost | $ 3,958,800 |  |
Other Investments | 315,800 | $ 331,900 |
Private equity funds |  |  |
Investment securities |  |  |
Other Investments | 82,100 | 45,300 |
Surplus Note |  |  |
Investment securities |  |  |
Other Investments | 51,500 | 65,100 |
Convertible fixed maturity investments |  |  |
Investment securities |  |  |
Other Investments | 0 | 5,600 |
Private equity securities |  |  |
Investment securities |  |  |
Other Investments | 32,700 | 8,300 |
Affordable Housing Development Fund [Member] |  |  |
Investment securities |  |  |
Other Investments | 14,700 | 16,800 |
Partnership investments [Member] |  |  |
Investment securities |  |  |
Other Investments | 3,800 | 5,200 |
Total hedge and private equity funds included in other long-term investments |  |  |
Investment securities |  |  |
Other Investments | 127,800 | 178,500 |
Other Debt Obligations [Member] |  |  |
Investment securities |  |  |
Other Investments | 3,200 | 7,100 |
Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments |  |  |
Investment securities |  |  |
Carrying value of investment accounted for using the equity method | $ 3,800 | $ 5,200 | $ 6,800

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