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Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost/Cost | $ 7,302,950 | $ 7,152,127
Amortized Cost/Cost | 116,320 | 134,013
Fair Value | 7,724,075 | 7,456,708
Fair Value | 135,466 | 141,649
Fixed maturity securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost/Cost | 7,302,950 | 7,152,127
Unrealized Gains | 443,633 | 374,991
Unrealized Losses | 22,508 | 70,410
Fair Value | 7,724,075 | 7,456,708
OTTI in AOCI | 0 | 1,618
Mortgage-backed securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost/Cost | 669,297 | 412,891
Unrealized Gains | 30,460 | 33,168
Unrealized Losses | 3,032 | 3,640
Fair Value | 696,725 | 442,419
OTTI in AOCI | 0 | 0
Other, including U.S. Treasury securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost/Cost | 714,613 | 458,745
Unrealized Gains | 26,311 | 18,518
Unrealized Losses | 5,516 | 10,120
Fair Value | 735,408 | 467,143
OTTI in AOCI | 0 | 0
Municipal bonds |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost/Cost | 1,711,581 | 1,648,252
Unrealized Gains | 184,107 | 143,733
Unrealized Losses | 2,435 | 22,588
Fair Value | 1,893,253 | 1,769,397
OTTI in AOCI | 0 | 0
Foreign government bonds |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost/Cost | 96,780 | 93,864
Unrealized Gains | 5,958 | 5,102
Unrealized Losses | 0 | 297
Fair Value | 102,738 | 98,669
OTTI in AOCI | 0 | 0
Corporate bonds |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost/Cost | 2,409,426 | 2,672,818
Unrealized Gains | 173,862 | 152,229
Unrealized Losses | 4,334 | 14,826
Fair Value | 2,578,954 | 2,810,221
OTTI in AOCI | 0 | 0
Other mortgage-backed securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost/Cost | 1,701,253 | 1,865,557
Unrealized Gains | 22,935 | 22,241
Unrealized Losses | 7,191 | 18,939
Fair Value | 1,716,997 | 1,868,859
OTTI in AOCI | 0 | 1,618
Equity securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost/Cost | 116,320 | 134,013
Unrealized Gains | 19,425 | 13,210
Unrealized Losses | 279 | 5,574
Fair Value | 135,466 | 141,649
OTTI in AOCI | 0 | 0
FNMA Obligations |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair Value | 361,955 | 196,468
FHLMC Obligations |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair Value | 400,001 | 284,050
GNMA Obligations |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair Value | $ 104,168 | $ 115,627

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