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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard updates						$ (663)	$ 2,948
Balance, beginning of period at Dec. 31, 2017			$ 233	$ (893,888)	$ 2,754,275	1,116,333	23,085
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1
Shares repurchased under share repurchase program (Note 13)			(3)		(50,050)
Repurchases of common stock under incentive plans				(747)
Issuance of common stock under incentive and benefit plans					2,593
Share-based compensation					13,808
Net income	$ 466,232					466,232
Dividends declared						(1,606)
Net unrealized gains (losses) on investments, net of tax	(85,276)						(85,276)
Net foreign currency translation adjustment, net of tax	3						3
Balance, end of period at Sep. 30, 2018		$ 3,347,278	231	(894,635)	2,720,626	1,580,296	(59,240)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard updates						0	0
Balance, beginning of period at Jun. 30, 2018			231	(894,610)	2,715,426	1,438,032	(57,943)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			0
Shares repurchased under share repurchase program (Note 13)			0		0
Repurchases of common stock under incentive plans				(25)
Issuance of common stock under incentive and benefit plans					1,014
Share-based compensation					4,186
Net income	142,797					142,797
Dividends declared						(533)
Net unrealized gains (losses) on investments, net of tax	(1,297)						(1,297)
Net foreign currency translation adjustment, net of tax	0						0
Balance, end of period at Sep. 30, 2018		3,347,278	231	(894,635)	2,720,626	1,580,296	(59,240)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard updates						0	0
Balance, beginning of period at Dec. 31, 2018	3,488,715		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 13)			(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 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	3,922,507	220	(901,556)	2,469,097	2,229,107	125,639
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard updates						0	0
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 13)			(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 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	$ 3,922,507	$ 220	$ (901,556)	$ 2,469,097	$ 2,229,107	$ 125,639