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Investments (Investments Held in a Loss Position) (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
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