Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities
Net income	$ 413,041	$ 555,713	$ 467,363
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on the sale of investments, net	(2,697)	(3,229)	(1,318)
Equity in net loss (income) of other invested assets	215	199	(111)
Distribution of income from other invested assets	1,277	648	211
Depreciation and amortization	3,328	3,769	3,404
Stock-based compensation expense	18,462	16,588	15,073
Amortization of premium on investment securities	23,393	15,738	14,220
Deferred income tax provision	38,653	59,481	50,692
Change in:
Accrued investment income	(1,413)	(908)	(4,820)
Accounts receivable	(8,848)	(3,767)	(7,118)
Deferred policy acquisition costs	(1,300)	344	(695)
Prepaid federal income tax	(40,751)	(59,500)	49,772
Other assets	(16,840)	(2,921)	(5,060)
Reserve for losses and LAE	305,579	19,898	2,614
Unearned premium reserve	(28,451)	(16,580)	35,795
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
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
Purchase of other invested assets	(17,012)	(49,789)	(30,860)
Return of investment from other invested assets	11,891	2,252	316
Purchase of property and equipment	(2,446)	(3,409)	(4,054)
Net cash used in investing activities	(1,154,417)	(545,076)	(546,905)
Financing Activities
Issuance of common shares, net of costs	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
Cash at beginning of year	71,350	64,946	43,524
Cash at end of year	102,830	71,350	64,946
Supplemental Disclosure of Cash Flow Information
Income tax payments	(38,705)	(41,505)	(29,275)
Interest payments	(8,263)	(9,863)	(9,451)
Noncash Transactions
Repayment of borrowings with term loan proceeds	(325,000)	0	(100,000)
Operating lease liabilities arising from obtaining right-of-use assets	$ 805	$ 1,334	$ 0