Consolidated Statements of Changes in Common Stockholders' Equity - USD ($) $ in Thousands	Total	Parent [Member]	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings/(Deficit)	Accumulated Other Comprehensive Income (Loss)
Cumulative effect of adopting accounting standard update					$ 756	$ (491)	$ 0
Balance, at Dec. 31, 2016			$ 232	$ (893,332)	2,779,891	997,890	(12,395)
Issuance of common stock under incentive and benefit plans			1		8,635
Stock-based compensation					13,491
Impact of extinguishment of convertible senior notes (Note 12)					(52,700)
Termination of capped calls (Note 12)					4,208
Shares repurchased under share repurchase program (Note 14)			0		(6)
Repurchases of common stock under incentive plans				(556)
Net income	$ 121,088					121,088
Dividends declared						(2,154)
Net unrealized gains (losses) on investments, net of tax	34,545						34,545
Net foreign currency translation adjustment, net of tax	871						871
Net actuarial gains (losses)	64						64
Balance, at Dec. 31, 2017		$ 3,000,038	233	(893,888)	2,754,275	1,116,333	23,085
Cumulative effect of adopting accounting standard update					0	(663)	2,948
Issuance of common stock under incentive and benefit plans			1		2,859
Stock-based compensation					17,649
Impact of extinguishment of convertible senior notes (Note 12)					0
Termination of capped calls (Note 12)					0
Shares repurchased under share repurchase program (Note 14)			(3)		(50,050)
Repurchases of common stock under incentive plans				(982)
Net income	606,011					606,011
Dividends declared						(2,140)
Net unrealized gains (losses) on investments, net of tax	(87,086)						(87,086)
Net foreign currency translation adjustment, net of tax	4						4
Net actuarial gains (losses)	129						129
Balance, at Dec. 31, 2018	3,488,715	3,488,715	231	(894,870)	2,724,733	1,719,541	(60,920)
Cumulative effect of adopting accounting standard update					0	0	0
Issuance of common stock under incentive and benefit plans			1		3,925
Stock-based compensation					21,414
Impact of extinguishment of convertible senior notes (Note 12)					0
Termination of capped calls (Note 12)					0
Shares repurchased under share repurchase program (Note 14)			(13)		(300,188)
Repurchases of common stock under incentive plans				(6,787)
Net income	672,309					672,309
Dividends declared						(2,061)
Net unrealized gains (losses) on investments, net of tax	171,544						171,544
Net foreign currency translation adjustment, net of tax	(3)						(3)
Net actuarial gains (losses)	(133)						(133)
Balance, at Dec. 31, 2019	$ 4,048,723	$ 4,048,723	$ 219	$ (901,657)	$ 2,449,884	$ 2,389,789	$ 110,488