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Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | $ 7,292,023 | $ 7,302,950
Amortized Cost/Cost |  | 116,320
Fair Value | 7,475,565 | 7,724,075
Federal National Mortgage Association Certificates and Obligations (FNMA) |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Fair Value | 384,676 | 361,955
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Fair Value | 395,043 | 400,001
Government National Mortgage Association Certificates and Obligations (GNMA) |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Fair Value | 97,103 | 104,168
Total Fixed Maturity Securities |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 7,292,023 | 7,302,950
Unrealized Investment Gains | 277,960 | 443,633
Unrealized Investment Losses | 94,418 | 22,508
Fair Value | 7,475,565 | 7,724,075
Mortgage-backed securities |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 704,330 | 669,297
Unrealized Investment Gains | 19,586 | 30,460
Unrealized Investment Losses | 16,592 | 3,032
Fair Value | 707,324 | 696,725
Other, including U.S. Treasury securities |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 790,687 | 714,613
Unrealized Investment Gains | 15,444 | 26,311
Unrealized Investment Losses | 23,908 | 5,516
Fair Value | 782,223 | 735,408
Municipal bonds |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 1,696,693 | 1,711,581
Unrealized Investment Gains | 141,937 | 184,107
Unrealized Investment Losses | 11,119 | 2,435
Fair Value | 1,827,511 | 1,893,253
Foreign government bonds |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 92,872 | 96,780
Unrealized Investment Gains | 3,217 | 5,958
Unrealized Investment Losses | 584 | 0
Fair Value | 95,505 | 102,738
Corporate bonds |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 2,342,531 | 2,409,426
Unrealized Investment Gains | 82,113 | 173,862
Unrealized Investment Losses | 26,920 | 4,334
Fair Value | 2,397,724 | 2,578,954
Other mortgage-backed securities |  |
Unrealized gains and losses on fixed maturities and equity securities |  |
Amortized Cost/Cost | 1,664,910 | 1,701,253
Unrealized Investment Gains | 15,663 | 22,935
Unrealized Investment Losses | 15,295 | 7,191
Fair Value | $ 1,665,278 | 1,716,997
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

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