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, 2022		$ 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)
Balance, beginning of period at Dec. 31, 2023	4,397,805	173	(945,870)	1,430,594	4,243,759	(330,851)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		0		1,531
Shares repurchased under share repurchase program (Note 14)		(2)		(50,501)
Repurchases of common stock under incentive plans			(332)
Share-based compensation				8,812
Net income	152,354				152,354
Dividends and dividend equivalents declared					(38,290)
Net unrealized gains (losses) on investments, net of tax	(31,577)					(31,577)
Other adjustments to other comprehensive income (loss)						(68)
Balance, end of period at Mar. 31, 2024	$ 4,439,732	$ 171	$ (946,202)	$ 1,390,436	$ 4,357,823	$ (362,496)