Investments - Fair Value and Gross Unrealized Losses (Details) $ in Thousands	Jun. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 457,895
Fixed maturity securities, Fair Value, More than 12 Months	426,861
Fixed maturity securities, Fair Value, Total	884,756
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	4,565
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	8,479
Fixed maturity securities, Gross Unrealized Losses, Total	$ 13,044
Number of positions with a gross unrealized loss, 12 Months or Less | security	143	1,052
Number of position with a gross unrealized loss, more than 12 months | security	156	359
Number of position with a gross unrealized loss, Total | security	299	1,411
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less	8.30%	33.20%
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months	7.70%	11.80%
Fair value as a percentage of total fixed maturities and equity securities fair value, Total	16.00%	45.00%
Mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 10,504	$ 193,447
Fixed maturity securities, Fair Value, More than 12 Months	44,564	157,295
Fixed maturity securities, Fair Value, Total	55,068	350,742
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	27	5,026
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	1,174	8,295
Fixed maturity securities, Gross Unrealized Losses, Total	1,201	13,321
Other
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	11,463	263,497
Fixed maturity securities, Fair Value, More than 12 Months	37,575	246,213
Fixed maturity securities, Fair Value, Total	49,038	509,710
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	32	6,746
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	501	10,935
Fixed maturity securities, Gross Unrealized Losses, Total	533	17,681
Municipal bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	16,900	291,869
Fixed maturity securities, Fair Value, More than 12 Months	34,031	95,297
Fixed maturity securities, Fair Value, Total	50,931	387,166
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	93	7,603
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	708	5,891
Fixed maturity securities, Gross Unrealized Losses, Total	801	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, 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	27,858	818,519
Fixed maturity securities, Fair Value, More than 12 Months	32,997	99,171
Fixed maturity securities, Fair Value, Total	60,855	917,690
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	518	27,429
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	1,693	11,462
Fixed maturity securities, Gross Unrealized Losses, Total	2,211	38,891
Other mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	391,170	913,858
Fixed maturity securities, Fair Value, More than 12 Months	277,694	291,442
Fixed maturity securities, Fair Value, Total	668,864	1,205,300
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	3,895	16,076
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	4,403	7,455
Fixed maturity securities, Gross Unrealized Losses, Total	$ 8,298	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