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

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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