Consolidated Statements of Changes in Common Stockholders' Equity - USD ($)	Total	Parent [Member]	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings/(Deficit)	Accumulated Other Comprehensive Income (Loss)
Balance, at Dec. 31, 2017			$ 233,000	$ (893,888,000)	$ 2,754,275,000	$ 1,116,333,000	$ 23,085,000
Issuance of common stock under incentive and benefit plans			1,000		2,859,000
Share-based compensation					17,649,000
Shares repurchased under share repurchase program (Note 14)			(3,000)		(50,050,000)
Repurchases of common stock under incentive plans				(982,000)
Cumulative effect of adopting accounting standards updates						(663,000)	2,948,000
Net income	$ 606,011,000					606,011,000
Dividends and dividend equivalents declared						(2,140,000)
Net unrealized gains (losses) on investments, net of tax	(87,086,000)						(87,086,000)
Other adjustments to comprehensive income, net	133,000						133,000
Balance, at Dec. 31, 2018		$ 3,488,715,000	231,000	(894,870,000)	2,724,733,000	1,719,541,000	(60,920,000)
Issuance of common stock under incentive and benefit plans			1,000		3,925,000
Share-based compensation					21,414,000
Shares repurchased under share repurchase program (Note 14)			(13,000)		(300,188,000)
Repurchases of common stock under incentive plans				(6,787,000)
Net income	672,309,000					672,309,000
Dividends and dividend equivalents declared						(2,061,000)
Net unrealized gains (losses) on investments, net of tax	171,544,000						171,544,000
Other adjustments to comprehensive income, net	(136,000)						(136,000)
Balance, at Dec. 31, 2019	4,048,723,000	4,048,723,000	219,000	(901,657,000)	2,449,884,000	2,389,789,000	110,488,000
Issuance of common stock under incentive and benefit plans			2,000		3,143,000
Share-based compensation					19,164,000
Shares repurchased under share repurchase program (Note 14)			(11,000)		(226,294,000)
Repurchases of common stock under incentive plans				(8,458,000)
Net income	393,626,000					393,626,000
Dividends and dividend equivalents declared						(98,779,000)
Net unrealized gains (losses) on investments, net of tax	153,312,000						153,312,000
Other adjustments to comprehensive income, net	(75,000)						(75,000)
Balance, at Dec. 31, 2020	$ 4,284,353,000	$ 4,284,353,000	$ 210,000	$ (910,115,000)	$ 2,245,897,000	$ 2,684,636,000	$ 263,725,000