Schedule II - Condensed Financial Information of Registrant - Cash Flow Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Jun. 30, 2020	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
Operating Activities
Net income		$ 123,602	$ 124,536	$ 15,380	$ 149,523	$ 146,958	$ 144,630	$ 136,405	$ 127,720	$ 413,041	$ 555,713	$ 467,363
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on the sale of investments, net										(2,697)	(3,229)	(1,318)
Stock-based compensation expense										18,462	16,588	15,073
Amortization of premium on investment securities										23,393	15,738	14,220
Changes in assets and liabilities:
Other assets										(16,840)	(2,921)	(5,060)
Other accrued liabilities										24,283	4,375	5,299
Net cash provided by operating activities										727,931	589,848	625,321
Investing Activities
Net change in short-term investments										(411,498)	(160,962)	158,294
Investments in subsidiaries										(17,012)	(49,789)	(30,860)
Purchase of investments available for sale										(1,575,082)	(1,044,964)	(1,126,032)
Proceeds from maturity of investments available for sale										262,321	88,025	114,651
Proceeds from sales of investments available for sale										577,409	623,771	340,780
Net cash used in investing activities										(1,154,417)	(545,076)	(546,905)
Financing Activities
Issuance of common shares, net of costs	$ 440,000									439,962	0	0
Credit facility borrowings										200,000	0	15,000
Credit facility repayments										(100,000)	0	(40,000)
Treasury stock acquired										(6,354)	(9,005)	(31,414)
Payment of issuance costs for credit facility										(6,232)	(15)	(580)
Dividends paid										(69,410)	(29,348)	0
Net cash provided by (used in) financing activities										457,966	(38,368)	(56,994)
Net increase in cash										31,480	6,404	21,422
Supplemental Disclosure of Cash Flow Information
Interest payments										(8,263)	(9,863)	(9,451)
Noncash Transactions
Repayment of borrowings with term loan proceeds										(325,000)	0	(100,000)
Parent Company
Operating Activities
Net income										413,041	555,713	467,363
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries										(427,101)	(569,183)	(478,880)
Loss on the sale of investments, net										10	0	0
Stock-based compensation expense										935	830	843
Amortization of premium on investment securities										435	0	0
Changes in assets and liabilities:
Other assets										(319)	416	911
Other accrued liabilities										18,208	15,562	15,537
Net cash provided by operating activities										5,209	3,338	5,774
Investing Activities
Net change in short-term investments										(255,884)	(24,727)	26,122
Investments in subsidiaries										0	55,001	0
Purchase of investments available for sale										(205,668)	0	0
Proceeds from maturity of investments available for sale										838	0	0
Proceeds from sales of investments available for sale										3,386	0	0
Net cash used in investing activities										(457,328)	30,274	26,122
Financing Activities
Issuance of common shares, net of costs										439,962	0	0
Credit facility borrowings										200,000	0	0
Credit facility repayments										(100,000)	0	0
Treasury stock acquired										(6,354)	(9,005)	(31,414)
Payment of issuance costs for credit facility										(5,236)	(15)	(131)
Dividends paid										(69,410)	(29,348)	0
Net cash provided by (used in) financing activities										458,962	(38,368)	(31,545)
Net increase in cash										6,843	(4,756)	351
Cash at beginning of year					$ 1,845				$ 6,601	1,845	6,601	6,250
Cash at end of year		$ 8,688				$ 1,845				8,688	1,845	6,601
Supplemental Disclosure of Cash Flow Information
Interest payments										(5,714)	(5,509)	(4,984)
Noncash Transactions
Repayment of borrowings with term loan proceeds										$ (225,000)	$ 0	$ 0