Investments - Gross Unrealized Losses of Fixed Maturities and Equity Securities in Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less		$ 910,409
Fixed maturity securities, Fair Value, More than 12 Months		448,829
Fixed maturity securities, Fair Value, Total		1,359,238
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less		8,743
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months		14,044
Fixed maturity securities, Gross Unrealized Losses, Total		$ 22,787
Number of positions with a gross unrealized loss position, 12 Months or Less | security	1,052	354
Number of position with a gross unrealized loss position, more than 12 months | security	359	158
Number of position with a gross unrealized loss position, Total | security	1,411	512
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less	32.70%	11.60%
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months	11.70%	5.70%
Fair value as a percentage of total fixed maturities and equity securities fair value, Total	44.40%	17.30%
Total
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 2,497,440	$ 904,382
Fixed maturity securities, Fair Value, More than 12 Months	889,418	447,552
Fixed maturity securities, Fair Value, Total	3,386,858	1,351,934
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	63,640	8,494
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	44,038	14,014
Fixed maturity securities, Gross Unrealized Losses, Total	107,678	22,508
Mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	193,447	134,032
Fixed maturity securities, Fair Value, More than 12 Months	157,295	40,606
Fixed maturity securities, Fair Value, Total	350,742	174,638
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	5,026	1,053
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	8,295	1,979
Fixed maturity securities, Gross Unrealized Losses, Total	13,321	3,032
Other
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	263,497	168,634
Fixed maturity securities, Fair Value, More than 12 Months	246,213	122,753
Fixed maturity securities, Fair Value, Total	509,710	291,387
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	6,746	1,849
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	10,935	3,667
Fixed maturity securities, Gross Unrealized Losses, Total	17,681	5,516
Municipal bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	291,869	29,437
Fixed maturity securities, Fair Value, More than 12 Months	95,297	79,140
Fixed maturity securities, Fair Value, Total	387,166	108,577
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	7,603	100
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	5,891	2,335
Fixed maturity securities, Gross Unrealized Losses, Total	13,494	2,435
Foreign government bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	16,250	0
Fixed maturity securities, Fair Value, More than 12 Months	0	0
Fixed maturity securities, Fair Value, Total	16,250	0
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	760	0
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0	0
Fixed maturity securities, Gross Unrealized Losses, Total	760	0
Corporate bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	818,519	115,113
Fixed maturity securities, Fair Value, More than 12 Months	99,171	36,081
Fixed maturity securities, Fair Value, Total	917,690	151,194
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	27,429	2,701
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	11,462	1,633
Fixed maturity securities, Gross Unrealized Losses, Total	38,891	4,334
Other mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	913,858	457,166
Fixed maturity securities, Fair Value, More than 12 Months	291,442	168,972
Fixed maturity securities, Fair Value, Total	1,205,300	626,138
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	16,076	2,791
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	7,455	4,400
Fixed maturity securities, Gross Unrealized Losses, Total	$ 23,531	7,191
Equity securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less		6,027
Fixed maturity securities, Fair Value, More than 12 Months		1,277
Fixed maturity securities, Fair Value, Total		7,304
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less		249
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months		30
Fixed maturity securities, Gross Unrealized Losses, Total		$ 279