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Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost/Cost | $ 7,373,911 | $ 7,302,950
Amortized Cost/Cost | 0 | 116,320
Fair Value | 7,515,318 | 7,724,075
Fair Value | 111,750 | 135,466
Fixed maturity securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost/Cost | 7,373,911 | 7,302,950
Unrealized Gains | 249,085 | 443,633
Unrealized Losses | 107,678 | 22,508
Fair Value | 7,515,318 | 7,724,075
OTTI in AOCI | 0 | 0
Mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost/Cost | 778,038 | 669,297
Unrealized Gains | 22,724 | 30,460
Unrealized Losses | 13,321 | 3,032
Fair Value | 787,441 | 696,725
OTTI in AOCI | 0 | 0
Other, including U.S. Treasury securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost/Cost | 835,096 | 714,613
Unrealized Gains | 16,127 | 26,311
Unrealized Losses | 17,681 | 5,516
Fair Value | 833,542 | 735,408
OTTI in AOCI | 0 | 0
Municipal bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost/Cost | 1,884,313 | 1,711,581
Unrealized Gains | 133,150 | 184,107
Unrealized Losses | 13,494 | 2,435
Fair Value | 2,003,969 | 1,893,253
OTTI in AOCI | 0 | 0
Foreign government bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost/Cost | 83,343 | 96,780
Unrealized Gains | 2,321 | 5,958
Unrealized Losses | 760 | 0
Fair Value | 84,904 | 102,738
OTTI in AOCI | 0 | 0
Corporate bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost/Cost | 2,054,105 | 2,409,426
Unrealized Gains | 64,296 | 173,862
Unrealized Losses | 38,891 | 4,334
Fair Value | 2,079,510 | 2,578,954
OTTI in AOCI | 0 | 0
Other mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost/Cost | 1,739,016 | 1,701,253
Unrealized Gains | 10,467 | 22,935
Unrealized Losses | 23,531 | 7,191
Fair Value | 1,725,952 | 1,716,997
OTTI in AOCI | 0 | 0
Equity securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost/Cost |  | 116,320
Unrealized Gains |  | 19,425
Unrealized Losses |  | 279
Fair Value |  | 135,466
OTTI in AOCI |  | 0
FNMA Obligations |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fair Value | 441,308 | 361,955
FHLMC Obligations |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fair Value | 417,308 | 400,001
GNMA Obligations |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fair Value | $ 96,466 | $ 104,168

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