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]
Issuance of common stock under incentive and benefit plans			1
Shares repurchased under share repurchase program (Note 14)			(12)		(275,173)
Repurchases of common stock under incentive plans				(6,686)
Issuance of common stock under incentive and benefit plans					4,418
Share-based compensation					15,119
Net income	$ 511,125					511,125
Dividends and dividend equivalents declared						(1,559)
Net unrealized gains (losses) on investments, net of tax	186,562						186,562
Net foreign currency translation adjustment, net of tax	(3)						(3)
Balance, end of period at Sep. 30, 2019		$ 3,922,507	220	(901,556)	2,469,097	2,229,107	125,639
Balance, beginning of period at Jun. 30, 2019			223	(901,419)	2,539,803	2,056,175	88,462
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			0
Shares repurchased under share repurchase program (Note 14)			(3)		(77,535)
Repurchases of common stock under incentive plans				(137)
Issuance of common stock under incentive and benefit plans					1,660
Share-based compensation					5,169
Net income	173,438					173,438
Dividends and dividend equivalents declared						(506)
Net unrealized gains (losses) on investments, net of tax	37,177						37,177
Net foreign currency translation adjustment, net of tax	0						0
Balance, end of period at Sep. 30, 2019		3,922,507	220	(901,556)	2,469,097	2,229,107	125,639
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]
Issuance of common stock under incentive and benefit plans			2
Shares repurchased under share repurchase program (Note 14)			(11)		(226,294)
Repurchases of common stock under incentive plans				(8,088)
Issuance of common stock under incentive and benefit plans					3,096
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						121,481
Net foreign currency translation adjustment, net of tax	0						0
Balance, end of period at Sep. 30, 2020	4,122,379	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
Shares repurchased under share repurchase program (Note 14)			0		0
Repurchases of common stock under incentive plans				(7)
Issuance of common stock under incentive and benefit plans					825
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						19,808
Net foreign currency translation adjustment, net of tax	0						0
Balance, end of period at Sep. 30, 2020	$ 4,122,379	$ 4,122,379	$ 210	$ (909,745)	$ 2,238,869	$ 2,561,076	$ 231,969