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Investments (Investments Held in a Loss Position) (Details) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
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Investments in LPs/LLCs [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value |  | $ 23,683
Unrealized Loss |  | 3,948
Less than 12 months, Fair Value |  | 22,265
Less than 12 months, Unrealized loss |  | 3,711
12 months or longer, Fair Value |  | 1,418
12 months or longer, Unrealized loss |  | 237
U.S. Treasury obligations [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | $ 66,685 | 61,209
Unrealized Loss | 658 | 987
Less than 12 months, Fair Value | 61,869 | 46,869
Less than 12 months, Unrealized loss | 591 | 617
12 months or longer, Fair Value | 4,816 | 14,340
12 months or longer, Unrealized loss | 67 | 370
U.S. Government-sponsored enterprise obligations [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 6,819 | 6,268
Unrealized Loss | 49 | 100
Less than 12 months, Fair Value | 6,819 | 2,775
Less than 12 months, Unrealized loss | 49 | 44
12 months or longer, Fair Value | 0 | 3,493
12 months or longer, Unrealized loss | 0 | 56
State and municipal bonds [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 46,193 | 39,831
Unrealized Loss | 823 | 335
Less than 12 months, Fair Value | 36,822 | 18,910
Less than 12 months, Unrealized loss | 703 | 84
12 months or longer, Fair Value | 9,371 | 20,921
12 months or longer, Unrealized loss | 120 | 251
Corporate debt [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 622,991 | 423,107
Unrealized Loss | 29,162 | 17,103
Less than 12 months, Fair Value | 555,097 | 326,804
Less than 12 months, Unrealized loss | 15,691 | 13,236
12 months or longer, Fair Value | 67,894 | 96,303
12 months or longer, Unrealized loss | 13,471 | 3,867
Residential mortgage-backed securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 87,567 | 45,006
Unrealized Loss | 1,311 | 448
Less than 12 months, Fair Value | 78,961 | 14,406
Less than 12 months, Unrealized loss | 1,095 | 31
12 months or longer, Fair Value | 8,606 | 30,600
12 months or longer, Unrealized loss | 216 | 417
Agency commercial mortgage-backed securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 409 | 4,783
Unrealized Loss | 26 | 59
Less than 12 months, Fair Value | 0 | 70
Less than 12 months, Unrealized loss | 0 | 0
12 months or longer, Fair Value | 409 | 4,713
12 months or longer, Unrealized loss | 26 | 59
Other commercial mortgage-backed securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 15,960 | 13,860
Unrealized Loss | 203 | 99
Less than 12 months, Fair Value | 12,635 | 7,005
Less than 12 months, Unrealized loss | 170 | 28
12 months or longer, Fair Value | 3,325 | 6,855
12 months or longer, Unrealized loss | 33 | 71
Other asset-backed securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 79,637 | 62,577
Unrealized Loss | 247 | 205
Less than 12 months, Fair Value | 74,150 | 59,176
Less than 12 months, Unrealized loss | 237 | 109
12 months or longer, Fair Value | 5,487 | 3,401
12 months or longer, Unrealized loss | 10 | 96
Debt Securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 926,261 | 656,641
Unrealized Loss | 32,479 | 19,336
Less than 12 months, Fair Value | 826,353 | 476,015
Less than 12 months, Unrealized loss | 18,536 | 14,149
12 months or longer, Fair Value | 99,908 | 180,626
12 months or longer, Unrealized loss | $ 13,943 | $ 5,187

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