Condensed Consolidated Statements of Changes in Common Stockholders' Equity (Unaudited) - 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, 2021		$ 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		0		1,363
Shares repurchased under share repurchase program (Note 14)		(1)		(21,328)
Repurchases of common stock under incentive plans			(160)
Share-based compensation				13,356
Net income	$ 181,131				181,131
Dividends and dividend equivalents declared					(35,947)
Net unrealized gains (losses) on investments, net of tax	(249,730)					(249,730)
Other adjustments to other comprehensive income (loss)						84
Balance, end of period at Mar. 31, 2022	4,147,564	193	(920,958)	1,871,763	3,326,119	(129,553)
Balance, beginning 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		1		2,105
Shares repurchased under share repurchase program (Note 14)		(1)		(15,156)
Repurchases of common stock under incentive plans			(670)
Share-based compensation				9,262
Net income	157,757				157,757
Dividends and dividend equivalents declared					(36,313)
Net unrealized gains (losses) on investments, net of tax	69,987					69,987
Other adjustments to other comprehensive income (loss)						179
Balance, end of period at Mar. 31, 2023	$ 4,106,478	$ 176	$ (931,313)	$ 1,515,852	$ 3,908,396	$ (386,633)