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)
Balance, beginning of period at Dec. 31, 2020		$ 210	$ (910,115)	$ 2,245,897	$ 2,684,636	$ 263,725
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		2		3,114
Shares repurchased under share repurchase programs (Note 14)		(18)		(399,082)
Repurchases of common stock under incentive plans			(10,683)
Share-based compensation				28,443
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		2		3,386
Shares repurchased under share repurchase programs (Note 14)		(20)		(400,175)
Repurchases of common stock under incentive plans			(9,845)
Share-based compensation				38,058
Net income	742,934				742,934
Dividends and dividend equivalents declared					(136,917)
Net unrealized gains (losses) on investments, net of tax	(576,976)					(576,976)
Other adjustments to other comprehensive income (loss)						84
Balance, end of period at Dec. 31, 2022	3,919,327	176	(930,643)	1,519,641	3,786,952	(456,799)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		2		4,173
Shares repurchased under share repurchase programs (Note 14)		(5)		(134,349)
Repurchases of common stock under incentive plans			(15,227)
Share-based compensation				41,129
Net income	603,119				603,119
Dividends and dividend equivalents declared					(146,312)
Net unrealized gains (losses) on investments, net of tax	125,769					125,769
Other adjustments to other comprehensive income (loss)						179
Balance, end of period at Dec. 31, 2023	$ 4,397,805	$ 173	$ (945,870)	$ 1,430,594	$ 4,243,759	$ (330,851)