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)	ASU 2016-09 Additional Paid-in Capital	ASU 2016-09 Retained Earnings/(Deficit)	ASU 2016-01 Retained Earnings/(Deficit)	ASU 2016-01 Accumulated Other Comprehensive Income (Loss)	ASU 2018-02 Retained Earnings/(Deficit)	ASU 2018-02 Accumulated Other Comprehensive Income (Loss)
Cumulative effect of adopting accounting standard update									$ 0	$ 0	$ 0	$ 0	$ 0
Balance, at Dec. 31, 2015			$ 224	$ (893,176)	$ 2,716,618	$ 691,742	$ (18,477)
Impact of extinguishment of convertible senior notes (Note 12)			17
Issuance of common stock under incentive and benefit plans			0		2,117
Termination of capped calls (Note 12)					0
Shares repurchased under share repurchase program (Note 15)			(9)		(100,179)
Repurchases of common stock under incentive plans				(156)
Stock-based compensation					18,257
Impact of extinguishment of convertible senior notes (Note 12)					143,078
Net income	$ 308,253					308,253
Dividends declared						(2,105)
Net unrealized gains (losses) on investments, net of tax	6,531						6,531
Net foreign currency translation adjustment, net of tax	(474)						(474)
Net actuarial gains	25						25
Balance, at Dec. 31, 2016		$ 2,872,286	232	(893,332)	2,779,891	997,890	(12,395)
Cumulative effect of adopting accounting standard update								$ 0	(491)	0	0	0	0
Impact of extinguishment of convertible senior notes (Note 12)			0
Issuance of common stock under incentive and benefit plans			1		8,635
Termination of capped calls (Note 12)					4,208
Shares repurchased under share repurchase program (Note 15)			0		(6)
Repurchases of common stock under incentive plans				(556)
Stock-based compensation					13,491
Impact of extinguishment of convertible senior notes (Note 12)					(52,700)
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	64						64
Balance, at Dec. 31, 2017	3,000,038	3,000,038	233	(893,888)	2,754,275	1,116,333	23,085
Cumulative effect of adopting accounting standard update								756	$ 0	$ 2,061	$ 224	$ (2,724)	$ 2,724
Impact of extinguishment of convertible senior notes (Note 12)			0
Issuance of common stock under incentive and benefit plans			1		2,859
Termination of capped calls (Note 12)					0
Shares repurchased under share repurchase program (Note 15)			(3)		(50,050)
Repurchases of common stock under incentive plans				(982)
Stock-based compensation					17,649
Impact of extinguishment of convertible senior notes (Note 12)					0
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	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