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, 2018			$ 231	$ (894,870)	$ 2,724,733	$ 1,719,541	$ (60,920)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased under share repurchase program (Note 13)			(1)		(31,773)
Repurchases of common stock under incentive plans				(451)
Issuance of common stock under incentive and benefit plans					1,069
Share-based compensation					3,695
Net income	$ 170,957					170,957
Dividends and dividend equivalents declared						(534)
Net unrealized gains (losses) on investments, net of tax	78,414						78,414
Balance, end of period at Mar. 31, 2019		$ 3,710,091	230	(895,321)	2,697,724	1,889,964	17,494
Balance, beginning of period at Dec. 31, 2019	4,048,723		219	(901,657)	2,449,884	2,389,789	110,488
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased under share repurchase program (Note 13)			(11)		(226,294)
Repurchases of common stock under incentive plans				(367)
Issuance of common stock under incentive and benefit plans					2,235
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)						(80,687)
Balance, end of period at Mar. 31, 2020	$ 3,864,508	$ 3,864,508	$ 208	$ (902,024)	$ 2,231,670	$ 2,504,853	$ 29,801