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		3,096
Shares repurchased under share repurchase program (Note 14)			(11)		(226,294)
Repurchases of common stock under incentive plans				(8,088)
Share-based compensation					12,183
Net income	$ 245,613					245,613
Dividends and dividend equivalents declared						(74,326)
Net unrealized gains (losses) on investments, net of tax							121,481
Balance, end of period at Sep. 30, 2020		$ 4,122,379	210	(909,745)	2,238,869	2,561,076	231,969
Balance, beginning of period at Jun. 30, 2020			210	(909,738)	2,232,949	2,450,423	212,161
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			0		825
Shares repurchased under share repurchase program (Note 14)			0		0
Repurchases of common stock under incentive plans				(7)
Share-based compensation					5,095
Net income	135,103					135,103
Dividends and dividend equivalents declared						(24,450)
Net unrealized gains (losses) on investments, net of tax							19,808
Balance, end of period at Sep. 30, 2020		4,122,379	210	(909,745)	2,238,869	2,561,076	231,969
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		2,944
Shares repurchased under share repurchase program (Note 14)			(11)		(256,967)
Repurchases of common stock under incentive plans				(10,240)
Share-based compensation					20,996
Net income	407,226					407,226
Dividends and dividend equivalents declared						(78,865)
Net unrealized gains (losses) on investments, net of tax							(111,501)
Balance, end of period at Sep. 30, 2021	4,257,936	4,257,936	200	(920,355)	2,012,870	3,012,997	152,224
Balance, beginning of period at Jun. 30, 2021			207	(920,225)	2,161,857	2,913,138	178,856
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			0		1,007
Shares repurchased under share repurchase program (Note 14)			(7)		(158,276)
Repurchases of common stock under incentive plans				(130)
Share-based compensation					8,282
Net income	126,412					126,412
Dividends and dividend equivalents declared						(26,553)
Net unrealized gains (losses) on investments, net of tax							(26,632)
Balance, end of period at Sep. 30, 2021	$ 4,257,936	$ 4,257,936	$ 200	$ (920,355)	$ 2,012,870	$ 3,012,997	$ 152,224