Investments - Schedule of Available-for-Sale Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total fixed maturity securities, Cost or Amortized Cost	$ 1,724,561	$ 1,398,533
Fixed maturity securities, available-for-sale	1,813,471	1,433,626
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Total fixed maturity securities, Cost or Amortized Cost		1,398,533
Total investments available-for-sale, Gross Unrealized Gains	91,529	37,691
Total investments available-for-sale, Gross Unrealized Losses	(2,619)	(2,598)
Fixed maturity securities, available-for-sale	1,813,471	1,433,626
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total fixed maturity securities, Cost or Amortized Cost	271,218	159,894
Total investments available-for-sale, Gross Unrealized Gains	17,484	7,949
Total investments available-for-sale, Gross Unrealized Losses	(106)	(742)
Fixed maturity securities, available-for-sale	288,596	167,101
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total fixed maturity securities, Cost or Amortized Cost	297,932	261,524
Total investments available-for-sale, Gross Unrealized Gains	8,591	3,244
Total investments available-for-sale, Gross Unrealized Losses	(54)	(622)
Fixed maturity securities, available-for-sale	306,469	264,146
Corporate
Debt Securities, Available-for-sale [Line Items]
Total fixed maturity securities, Cost or Amortized Cost	727,678	611,304
Total investments available-for-sale, Gross Unrealized Gains	50,067	21,306
Total investments available-for-sale, Gross Unrealized Losses	(790)	(389)
Fixed maturity securities, available-for-sale	776,955	632,221
Commercial mortgage and asset-backed
Debt Securities, Available-for-sale [Line Items]
Total fixed maturity securities, Cost or Amortized Cost	324,966	249,309
Total investments available-for-sale, Gross Unrealized Gains	12,535	3,954
Total investments available-for-sale, Gross Unrealized Losses	(1,667)	(806)
Fixed maturity securities, available-for-sale	335,834	252,457
U.S. Treasury securities and obligations guaranteed by the U.S. government
Debt Securities, Available-for-sale [Line Items]
Total fixed maturity securities, Cost or Amortized Cost	100,742	114,477
Total investments available-for-sale, Gross Unrealized Gains	2,852	1,229
Total investments available-for-sale, Gross Unrealized Losses	0	(39)
Fixed maturity securities, available-for-sale	103,594	115,667
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Total fixed maturity securities, Cost or Amortized Cost	2,025	2,025
Total investments available-for-sale, Gross Unrealized Gains	0	9
Total investments available-for-sale, Gross Unrealized Losses	(2)	0
Fixed maturity securities, available-for-sale	$ 2,023	$ 2,034