Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	$ 5,241,755	$ 7,373,911
Fair Value	5,534,270	7,515,318
Federal National Mortgage Association Certificates and Obligations (FNMA)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	370,182	441,308
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	256,531	417,308
Government National Mortgage Association Certificates and Obligations (GNMA)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	79,141	96,466
Total Fixed Maturity Securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	5,241,755	7,373,911
Unrealized Gains	305,559	249,085
Unrealized Losses	13,044	107,678
Fair Value	5,534,270	7,515,318
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	550,059	778,038
Unrealized Gains	37,900	22,724
Unrealized Losses	1,201	13,321
Fair Value	586,758	787,441
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	546,396	835,096
Unrealized Gains	20,574	16,127
Unrealized Losses	533	17,681
Fair Value	566,437	833,542
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,472,794	1,884,313
Unrealized Gains	127,700	133,150
Unrealized Losses	801	13,494
Fair Value	1,599,693	2,003,969
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	45,303	83,343
Unrealized Gains	2,064	2,321
Unrealized Losses	0	760
Fair Value	47,367	84,904
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,226,962	2,054,105
Unrealized Gains	96,535	64,296
Unrealized Losses	2,211	38,891
Fair Value	1,321,286	2,079,510
Other mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,400,241	1,739,016
Unrealized Gains	20,786	10,467
Unrealized Losses	8,298	23,531
Fair Value	$ 1,412,729	$ 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=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

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-Publisher FASB
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-Topic 320
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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