Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,788,551	$ 5,456,980
Fair Value	6,345,290	5,791,676
FNMA
Debt Securities, Available-for-sale [Line Items]
Fair Value	387,100	405,100
FHLMC
Debt Securities, Available-for-sale [Line Items]
Fair Value	344,300	283,100
GNMA
Debt Securities, Available-for-sale [Line Items]
Fair Value	132,300	147,400
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,788,551	5,456,980
Unrealized Gains	585,121	348,310
Unrealized Losses	28,382	13,614
Fair Value	6,345,290	5,791,676
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	605,468	684,543
Unrealized Gains	79,601	41,263
Unrealized Losses	231	1,487
Fair Value	684,838	724,319
Other, including U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	395,042	436,665
Unrealized Gains	39,144	22,824
Unrealized Losses	1,033	621
Fair Value	433,153	458,868
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,612,290	1,545,787
Unrealized Gains	215,711	141,996
Unrealized Losses	504	1,580
Fair Value	1,827,497	1,686,203
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	40,145	42,801
Unrealized Gains	4,908	2,569
Unrealized Losses	0	0
Fair Value	45,053	45,370
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,905,207	1,464,444
Unrealized Gains	221,634	118,775
Unrealized Losses	3,942	1,795
Fair Value	2,122,899	1,581,424
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,230,399	1,282,740
Unrealized Gains	24,123	20,883
Unrealized Losses	22,672	8,131
Fair Value	$ 1,231,850	$ 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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