Investments - Fair Value and Gross Unrealized Losses (Details) $ in Thousands	Sep. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 425,864
Fixed maturity securities, Fair Value, More than 12 Months	328,227
Fixed maturity securities, Fair Value, Total	754,091
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	5,051
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	5,604
Fixed maturity securities, Gross Unrealized Losses, Total	$ 10,655
Number of positions with a gross unrealized loss, 12 Months or Less \| security	214	1,052
Number of position with a gross unrealized loss, more than 12 months \| security	102	359
Number of position with a gross unrealized loss, Total \| security	316	1,411
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less	7.30%	33.20%
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months	5.60%	11.80%
Fair value as a percentage of total fixed maturities and equity securities fair value, Total	12.90%	45.00%
Mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 21,323	$ 193,447
Fixed maturity securities, Fair Value, More than 12 Months	2,795	157,295
Fixed maturity securities, Fair Value, Total	24,118	350,742
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	204	5,026
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	103	8,295
Fixed maturity securities, Gross Unrealized Losses, Total	307	13,321
Other
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	12,712	263,497
Fixed maturity securities, Fair Value, More than 12 Months	2,024	246,213
Fixed maturity securities, Fair Value, Total	14,736	509,710
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	128	6,746
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	7	10,935
Fixed maturity securities, Gross Unrealized Losses, Total	135	17,681
Municipal bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	11,707	291,869
Fixed maturity securities, Fair Value, More than 12 Months	13,185	95,297
Fixed maturity securities, Fair Value, Total	24,892	387,166
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	74	7,603
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	282	5,891
Fixed maturity securities, Gross Unrealized Losses, Total	356	13,494
Foreign government bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	2,500	16,250
Fixed maturity securities, Fair Value, More than 12 Months	0	0
Fixed maturity securities, Fair Value, Total	2,500	16,250
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	2	760
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0	0
Fixed maturity securities, Gross Unrealized Losses, Total	2	760
Corporate bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	57,329	818,519
Fixed maturity securities, Fair Value, More than 12 Months	16,371	99,171
Fixed maturity securities, Fair Value, Total	73,700	917,690
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	1,145	27,429
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	825	11,462
Fixed maturity securities, Gross Unrealized Losses, Total	1,970	38,891
Other mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	320,293	913,858
Fixed maturity securities, Fair Value, More than 12 Months	293,852	291,442
Fixed maturity securities, Fair Value, Total	614,145	1,205,300
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	3,498	16,076
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	4,387	7,455
Fixed maturity securities, Gross Unrealized Losses, Total	$ 7,885	23,531
Total Fixed Maturity Securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less		2,497,440
Fixed maturity securities, Fair Value, More than 12 Months		889,418
Fixed maturity securities, Fair Value, Total		3,386,858
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less		63,640
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months		44,038
Fixed maturity securities, Gross Unrealized Losses, Total		$ 107,678

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