Condensed Consolidated Statements of Changes in Common Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Parent	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					1,167
Shares repurchased under share repurchase program (Note 14)			0		(8,637)
Repurchases of common stock under incentive plans				(232)
Share-based compensation					4,523
Net income	$ 125,608					125,608
Dividends and dividend equivalents declared						(24,500)
Net unrealized gains (losses) on investments	(146,990)						(146,990)
Other adjustments to other comprehensive income (loss)							0
Balance, end of period at Mar. 31, 2021		$ 4,235,292	210	(910,347)	2,242,950	2,785,744	116,735
Balance, beginning 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					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	(249,730)						(249,730)
Other adjustments to other comprehensive income (loss)							84
Balance, end of period at Mar. 31, 2022	$ 4,147,564	$ 4,147,564	$ 193	$ (920,958)	$ 1,871,763	$ 3,326,119	$ (129,553)