Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	$ 7,487,473	$ 7,373,911
Fair Value	7,798,004	7,515,318
Federal National Mortgage Association Certificates and Obligations (FNMA)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	474,541	441,308
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	421,933	417,308
Government National Mortgage Association Certificates and Obligations (GNMA)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	96,844	96,466
Total Fixed Maturity Securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	7,487,473	7,373,911
Unrealized Gains	350,665	249,085
Unrealized Losses	40,134	107,678
Fair Value	7,798,004	7,515,318
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	793,290	778,038
Unrealized Gains	32,299	22,724
Unrealized Losses	5,402	13,321
Fair Value	820,187	787,441
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	848,659	835,096
Unrealized Gains	23,333	16,127
Unrealized Losses	6,534	17,681
Fair Value	865,458	833,542
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,915,504	1,884,313
Unrealized Gains	171,412	133,150
Unrealized Losses	3,872	13,494
Fair Value	2,083,044	2,003,969
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	83,330	83,343
Unrealized Gains	3,084	2,321
Unrealized Losses	179	760
Fair Value	86,235	84,904
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	2,041,390	2,054,105
Unrealized Gains	104,303	64,296
Unrealized Losses	11,601	38,891
Fair Value	2,134,092	2,079,510
Other mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,805,300	1,739,016
Unrealized Gains	16,234	10,467
Unrealized Losses	12,546	23,531
Fair Value	$ 1,808,988	$ 1,725,952

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
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
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