Schedule II - Condensed Financial Information of Registrant - Comprehensive Income Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Net investment income									$ 80,087	$ 83,542	$ 64,091
Realized investment losses, net									2,697	3,229	1,318
Total revenues									955,154	867,567	719,353
Expenses:
Interest expense	$ 2,149	$ 2,227	$ 2,566	$ 2,132	$ 2,218	$ 2,584	$ 2,679	$ 2,670	9,074	10,151	10,179
Income before income taxes	146,152	148,431	18,815	176,698	174,384	172,225	162,733	149,719	490,096	659,061	546,699
Net income	$ 123,602	$ 124,536	$ 15,380	$ 149,523	$ 146,958	$ 144,630	$ 136,405	$ 127,720	413,041	555,713	467,363
Other comprehensive income (loss):
Change in unrealized appreciation (depreciation) of investments, net of tax expense (benefit) of $16,836 in 2020, $17,497 in 2019 and $(5,686) in 2018									82,087	85,180	(25,741)
Total other comprehensive income (loss)									82,087	85,180	(25,741)
Comprehensive income									495,128	640,893	441,622
Parent Company
Revenues:
Net investment income									1,181	1,601	980
Realized investment losses, net									(10)	0	0
Administrative service fees from subsidiaries									872	649	551
Total revenues									2,043	2,250	1,531
Expenses:
Administrative service fees to subsidiaries									3,728	3,053	2,510
Other operating expenses									5,929	6,925	5,076
Interest expense									6,446	5,742	5,462
Total expenses									16,103	15,720	13,048
Income before income taxes									(14,060)	(13,470)	(11,517)
Loss before equity in undistributed net income of subsidiaries									(14,060)	(13,470)	(11,517)
Equity in undistributed net income of subsidiaries									427,101	569,183	478,880
Net income									413,041	555,713	467,363
Other comprehensive income (loss):
Change in unrealized appreciation (depreciation) of investments, net of tax expense (benefit) of $16,836 in 2020, $17,497 in 2019 and $(5,686) in 2018									82,087	85,180	(25,741)
Total other comprehensive income (loss)									82,087	85,180	(25,741)
Comprehensive income									495,128	640,893	441,622
Change in unrealized appreciation (depreciation) of investments, (benefit) tax expense									$ 16,836	$ 17,497	$ (5,686)