Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	$ 5,653,466	$ 5,456,980
Fair Value	5,843,183	5,791,676
FNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	413,300	405,100
FHLMC
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	308,400	283,100
GNMA
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	151,100	147,400
Total Fixed Maturity Securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	5,653,466	5,456,980
Unrealized Gains	351,271	348,310
Unrealized Losses	161,554	13,614
Fair Value	5,843,183	5,791,676
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	676,322	684,543
Unrealized Gains	75,985	41,263
Unrealized Losses	392	1,487
Fair Value	751,915	724,319
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	430,193	436,665
Unrealized Gains	38,540	22,824
Unrealized Losses	60	621
Fair Value	468,673	458,868
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,626,005	1,545,787
Unrealized Gains	151,302	141,996
Unrealized Losses	7,376	1,580
Fair Value	1,769,931	1,686,203
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	42,802	42,801
Unrealized Gains	1,804	2,569
Unrealized Losses	754	0
Fair Value	43,852	45,370
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,649,405	1,464,444
Unrealized Gains	77,572	118,775
Unrealized Losses	41,204	1,795
Fair Value	1,685,773	1,581,424
Other mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost	1,228,739	1,282,740
Unrealized Gains	6,068	20,883
Unrealized Losses	111,768	8,131
Fair Value	$ 1,123,039	$ 1,295,492

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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

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