Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,456,980	$ 7,373,911
Fair Value	5,791,676	7,515,318
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,456,980	7,373,911
Unrealized Gains	348,310	249,085
Unrealized Losses	13,614	107,678
Fair Value	5,791,676	7,515,318
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	684,543	778,038
Unrealized Gains	41,263	22,724
Unrealized Losses	1,487	13,321
Fair Value	724,319	787,441
Other, including U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	436,665	835,096
Unrealized Gains	22,824	16,127
Unrealized Losses	621	17,681
Fair Value	458,868	833,542
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,545,787	1,884,313
Unrealized Gains	141,996	133,150
Unrealized Losses	1,580	13,494
Fair Value	1,686,203	2,003,969
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	42,801	83,343
Unrealized Gains	2,569	2,321
Unrealized Losses	0	760
Fair Value	45,370	84,904
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,464,444	2,054,105
Unrealized Gains	118,775	64,296
Unrealized Losses	1,795	38,891
Fair Value	1,581,424	2,079,510
Other mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,282,740	1,739,016
Unrealized Gains	20,883	10,467
Unrealized Losses	8,131	23,531
Fair Value	1,295,492	1,725,952
FNMA
Debt Securities, Available-for-sale [Line Items]
Fair Value	405,100	441,300
FHLMC
Debt Securities, Available-for-sale [Line Items]
Fair Value	283,100	417,300
GNMA
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 147,400	$ 96,500

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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