Investments - Gross Unrealized Losses of Fixed Maturities and Equity Securities in Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 478,866	$ 2,497,440
Fixed maturity securities, Fair Value, More than 12 Months	472,720	889,418
Fixed maturity securities, Fair Value, Total	951,586	3,386,858
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	5,734	63,640
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	7,880	44,038
Fixed maturity securities, Gross Unrealized Losses, Total	$ 13,614	$ 107,678
Number of positions with a gross unrealized loss, 12 Months or Less \| security	330	1,052
Number of position with a gross unrealized loss, more than 12 months \| security	137	359
Number of position with a gross unrealized loss, Total \| security	467	1,411
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less	8.30%	32.70%
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months	8.20%	11.70%
Fair value as a percentage of total fixed maturities and equity securities fair value, Total	16.50%	44.40%
Mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 72,422	$ 193,447
Fixed maturity securities, Fair Value, More than 12 Months	2,620	157,295
Fixed maturity securities, Fair Value, Total	75,042	350,742
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	1,282	5,026
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	205	8,295
Fixed maturity securities, Gross Unrealized Losses, Total	1,487	13,321
Other
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	38,341	263,497
Fixed maturity securities, Fair Value, More than 12 Months	1,527	246,213
Fixed maturity securities, Fair Value, Total	39,868	509,710
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	619	6,746
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	2	10,935
Fixed maturity securities, Gross Unrealized Losses, Total	621	17,681
Municipal bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	91,195	291,869
Fixed maturity securities, Fair Value, More than 12 Months	9,160	95,297
Fixed maturity securities, Fair Value, Total	100,355	387,166
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	977	7,603
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	603	5,891
Fixed maturity securities, Gross Unrealized Losses, Total	1,580	13,494
Foreign government bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	0	16,250
Fixed maturity securities, Fair Value, More than 12 Months	0	0
Fixed maturity securities, Fair Value, Total	0	16,250
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	0	760
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0	0
Fixed maturity securities, Gross Unrealized Losses, Total	0	760
Corporate bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	58,198	818,519
Fixed maturity securities, Fair Value, More than 12 Months	16,622	99,171
Fixed maturity securities, Fair Value, Total	74,820	917,690
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	886	27,429
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	909	11,462
Fixed maturity securities, Gross Unrealized Losses, Total	1,795	38,891
Other mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	218,710	913,858
Fixed maturity securities, Fair Value, More than 12 Months	442,791	291,442
Fixed maturity securities, Fair Value, Total	661,501	1,205,300
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	1,970	16,076
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	6,161	7,455
Fixed maturity securities, Gross Unrealized Losses, Total	$ 8,131	$ 23,531

Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]

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Debt Securities, Available-for-sale, Unrealized Loss Position Abstract [Abstract]

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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