Investments - Fair Value and Gross Unrealized Losses (Details) $ in Thousands	Mar. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 755,551
Fixed maturity securities, Fair Value, More than 12 Months	1,204,178
Fixed maturity securities, Fair Value, Total	1,959,729
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	9,779
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	30,355
Fixed maturity securities, Gross Unrealized Losses, Total	$ 40,134
Number of positions with a gross unrealized loss, 12 Months or Less | security	234	1,052
Number of position with a gross unrealized loss, more than 12 months | security	496	359
Number of position with a gross unrealized loss, Total | security	730	1,411
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less	9.60%	32.70%
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months	15.20%	11.70%
Fair value as a percentage of total fixed maturities and equity securities fair value, Total	24.80%	44.40%
Mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 4,364	$ 193,447
Fixed maturity securities, Fair Value, More than 12 Months	218,612	157,295
Fixed maturity securities, Fair Value, Total	222,976	350,742
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	49	5,026
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	5,353	8,295
Fixed maturity securities, Gross Unrealized Losses, Total	5,402	13,321
Other
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	8,230	263,497
Fixed maturity securities, Fair Value, More than 12 Months	303,498	246,213
Fixed maturity securities, Fair Value, Total	311,728	509,710
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	16	6,746
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	6,518	10,935
Fixed maturity securities, Gross Unrealized Losses, Total	6,534	17,681
Municipal bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	15,003	291,869
Fixed maturity securities, Fair Value, More than 12 Months	127,688	95,297
Fixed maturity securities, Fair Value, Total	142,691	387,166
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	254	7,603
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	3,618	5,891
Fixed maturity securities, Gross Unrealized Losses, Total	3,872	13,494
Foreign government bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	7,925	16,250
Fixed maturity securities, Fair Value, More than 12 Months	6,416	0
Fixed maturity securities, Fair Value, Total	14,341	16,250
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	59	760
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	120	0
Fixed maturity securities, Gross Unrealized Losses, Total	179	760
Corporate bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	89,954	818,519
Fixed maturity securities, Fair Value, More than 12 Months	215,797	99,171
Fixed maturity securities, Fair Value, Total	305,751	917,690
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	808	27,429
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	10,793	11,462
Fixed maturity securities, Gross Unrealized Losses, Total	11,601	38,891
Other mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	630,075	913,858
Fixed maturity securities, Fair Value, More than 12 Months	332,167	291,442
Fixed maturity securities, Fair Value, Total	962,242	1,205,300
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	8,593	16,076
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	3,953	7,455
Fixed maturity securities, Gross Unrealized Losses, Total	$ 12,546	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