Schedule II - Condensed Financial Information of Registrant - Balance Sheet Information (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assets
Total investments available for sale	$ 4,565,373	$ 3,350,747
Cash	102,830	71,350
Other assets	40,793	18,367
Total assets	5,202,724	3,873,425
Liabilities
Credit facility borrowings (at carrying value, less unamortized deferred costs of $2,287 in 2020 and $564 in 2019)	321,720	224,237
Other accrued liabilities	87,885	66,474
Total liabilities	1,340,091	888,580
Commitments and contingencies
Stockholders' Equity
Common shares	1,686	1,476
Additional paid-in capital	1,571,163	1,118,655
Accumulated other comprehensive income	138,274	56,187
Retained earnings	2,151,510	1,808,527
Total stockholders' equity	3,862,633	2,984,845	$ 2,365,717
Total liabilities and stockholders' equity	5,202,724	3,873,425
Amortized Cost	4,404,690	3,282,585
Unamortized deferred costs	3,280	763
Fixed maturities
Assets
Total investments available for sale	3,838,513	3,035,385
Stockholders' Equity
Amortized Cost	3,677,815	2,967,225
Short-term investments
Assets
Total investments available for sale	726,860	315,362
Stockholders' Equity
Amortized Cost	726,875	315,360
Parent Company
Assets
Total investments available for sale	554,544	96,531
Cash	8,688	1,845
Due from affiliates	1,278	875
Investment in consolidated subsidiaries	3,518,029	3,009,447
Other assets	4,512	1,608
Total assets	4,087,051	3,110,306
Liabilities
Due to affiliates	0	301
Credit facility borrowings (at carrying value, less unamortized deferred costs of $2,287 in 2020 and $564 in 2019)	222,713	124,436
Other accrued liabilities	1,705	724
Total liabilities	224,418	125,461
Commitments and contingencies
Stockholders' Equity
Common shares	1,686	1,476
Additional paid-in capital	1,571,163	1,118,655
Accumulated other comprehensive income	138,274	56,187
Retained earnings	2,151,510	1,808,527
Total stockholders' equity	3,862,633	2,984,845
Total liabilities and stockholders' equity	4,087,051	3,110,306
Unamortized deferred costs	2,287	564
Parent Company | Fixed maturities
Assets
Total investments available for sale	202,129	0
Stockholders' Equity
Amortized Cost	201,527	0
Parent Company | Short-term investments
Assets
Total investments available for sale	352,415	96,531
Stockholders' Equity
Amortized Cost	$ 352,412	$ 96,531