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
Balance, beginning of period at Dec. 31, 2014			$ 209	$ (892,961)	$ 2,531,513	$ 406,814	$ 51,485
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1		2,394
Stock-based compensation					13,214
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)			28		349,191
Termination of capped calls (Note 11)			(3)		11,976
Change in equity component of currently redeemable convertible senior notes					11,911
Shares repurchased under share repurchase program (Note 13)			(11)		(201,989)
Repurchases of common stock under incentive plans				(215)
Net income	$ 212,396					212,396
Dividends declared						(1,479)
Net foreign currency translation adjustment, net of tax	(88)						(88)
Net unrealized gains (losses) on investments, net of tax							(58,816)
Net actuarial gains (losses)	0						0
Balance, end of period at Sep. 30, 2015		$ 2,435,570	224	(893,176)	2,718,210	617,731	(7,419)
Balance, beginning of period at Dec. 31, 2015	2,496,931		224	(893,176)	2,716,618	691,742	(18,477)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			0		1,711
Stock-based compensation					17,632
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)			17		143,078
Termination of capped calls (Note 11)			0		0
Change in equity component of currently redeemable convertible senior notes					0
Shares repurchased under share repurchase program (Note 13)			(9)		(100,179)
Repurchases of common stock under incentive plans				(21)
Net income	247,164					247,164
Dividends declared						(1,568)
Net foreign currency translation adjustment, net of tax	(346)						(346)
Net unrealized gains (losses) on investments, net of tax							84,318
Net actuarial gains (losses)	(22)						(22)
Balance, end of period at Sep. 30, 2016	$ 2,888,706	$ 2,888,706	$ 232	$ (893,197)	$ 2,778,860	$ 937,338	$ 65,473