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Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | $ 7,373,552 | $ 7,302,950
Amortized Cost/Cost |  | 116,320
Fair Value | 7,634,682 | 7,724,075
Fair Value | 131,023 | 135,466
Federal National Mortgage Association Certificates and Obligations (FNMA) |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Fair Value | 386,398 | 361,955
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Fair Value | 396,957 | 400,001
Government National Mortgage Association Certificates and Obligations (GNMA) |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Fair Value | 98,056 | 104,168
Total Fixed Maturity Securities |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 7,373,552 | 7,302,950
Unrealized Investment Gains | 324,880 | 443,633
Unrealized Investment Losses | 63,750 | 22,508
Fair Value | 7,634,682 | 7,724,075
OTTI in AOCI | 0 | 0
Mortgage-backed securities |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 699,629 | 669,297
Unrealized Investment Gains | 22,930 | 30,460
Unrealized Investment Losses | 11,601 | 3,032
Fair Value | 710,958 | 696,725
OTTI in AOCI | 0 | 0
Other, including U.S. Treasury securities |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 760,390 | 714,613
Unrealized Investment Gains | 18,797 | 26,311
Unrealized Investment Losses | 17,220 | 5,516
Fair Value | 761,967 | 735,408
OTTI in AOCI | 0 | 0
Municipal bonds |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 1,721,046 | 1,711,581
Unrealized Investment Gains | 149,774 | 184,107
Unrealized Investment Losses | 9,516 | 2,435
Fair Value | 1,861,304 | 1,893,253
OTTI in AOCI | 0 | 0
Foreign government bonds |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 94,870 | 96,780
Unrealized Investment Gains | 4,307 | 5,958
Unrealized Investment Losses | 136 | 0
Fair Value | 99,041 | 102,738
OTTI in AOCI | 0 | 0
Corporate bonds |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 2,383,394 | 2,409,426
Unrealized Investment Gains | 112,451 | 173,862
Unrealized Investment Losses | 14,575 | 4,334
Fair Value | 2,481,270 | 2,578,954
OTTI in AOCI | 0 | 0
Other mortgage-backed securities |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 1,714,223 | 1,701,253
Unrealized Investment Gains | 16,621 | 22,935
Unrealized Investment Losses | 10,702 | 7,191
Fair Value | 1,720,142 | 1,716,997
OTTI in AOCI | $ 0 | 0
Equity securities |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost |  | 116,320
Unrealized Investment Gains |  | 19,425
Unrealized Investment Losses |  | 279
Fair Value |  | 135,466
OTTI in AOCI |  | $ 0

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