Consolidated Statements of Changes in Common Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Total stockholders’ equity		$ 231	$ (894,870)	$ 2,724,733	$ 1,719,541	$ (60,920)
Balance, beginning of period at Dec. 31, 2018		231	(894,870)	2,724,733	1,719,541	(60,920)
Issuance of common stock under incentive and benefit plans		1		3,925
Share-based compensation				21,414
Shares repurchased under share repurchase program (Note 14)		(13)
Shares repurchased under incentive plan and repurchase program			(6,787)	(300,188)
Net income	$ 672,309				672,309
Dividends and dividend equivalents declared					(2,061)
Net unrealized gains (losses) on investments, net of tax	171,544					171,544
Other adjustments to other comprehensive income (loss)						(136)
Balance, end of period at Dec. 31, 2019	4,048,723	219	(901,657)	2,449,884	2,389,789	110,488
Total stockholders’ equity	4,048,723	219	(901,657)	2,449,884	2,389,789	110,488
Issuance of common stock under incentive and benefit plans		2		3,143
Share-based compensation				19,164
Shares repurchased under share repurchase program (Note 14)		(11)
Shares repurchased under incentive plan and repurchase program			(8,458)	(226,294)
Net income	393,626				393,626
Dividends and dividend equivalents declared					(98,779)
Net unrealized gains (losses) on investments, net of tax	153,312					153,312
Other adjustments to other comprehensive income (loss)						(75)
Balance, end of period at Dec. 31, 2020	4,284,353	210	(910,115)	2,245,897	2,684,636	263,725
Total stockholders’ equity	4,284,353	210	(910,115)	2,245,897	2,684,636	263,725
Issuance of common stock under incentive and benefit plans		2		3,114
Share-based compensation				28,443
Shares repurchased under share repurchase program (Note 14)		(18)
Shares repurchased under incentive plan and repurchase program			(10,683)	(399,082)
Net income	600,671				600,671
Dividends and dividend equivalents declared					(104,372)
Net unrealized gains (losses) on investments, net of tax	(143,632)					(143,632)
Other adjustments to other comprehensive income (loss)						0
Balance, end of period at Dec. 31, 2021	4,258,796	194	(920,798)	1,878,372	3,180,935	120,093
Total stockholders’ equity	$ 4,258,796	$ 194	$ (920,798)	$ 1,878,372	$ 3,180,935	$ 120,093