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, 2019			$ 219	$ (901,657)	$ 2,449,884	$ 2,389,789	$ 110,488
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans					2,235
Stock Repurchased During Period, Value			(11)		(226,294)
Repurchases of common stock under incentive plans				(367)
Share-based compensation					5,845
Net income	$ 140,461					140,461
Dividends and dividend equivalents declared						(25,397)
Net unrealized gains (losses) on investments, net of tax							(80,687)
Balance, end of period at Mar. 31, 2020		$ 3,864,508	208	(902,024)	2,231,670	2,504,853	29,801
Balance, beginning of period at Dec. 31, 2020	4,284,353		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
Stock Repurchased During Period, Value			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, net of tax							(146,990)
Balance, end of period at Mar. 31, 2021	$ 4,235,292	$ 4,235,292	$ 210	$ (910,347)	$ 2,242,950	$ 2,785,744	$ 116,735