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, 2015			$ 224	$ (893,176)	$ 2,716,618	$ 691,742	$ (18,477)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact to common stock related to extinguishment of Convertible Senior Notes			17
Issuance of common stock under incentive and benefit plans			0
Stock Repurchased and Retired During Period, Value			(9)		(100,179)
Share-based compensation					14,428
Issuance of common stock under incentive and benefit plans					726
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)					149,543
Equity Impact of Termination of Capped Call Transaction					0
Change in equity component of currently redeemable convertible senior notes					0
Repurchases of common stock under incentive plans				0
Net income (loss)	$ 164,361					164,361
Dividends declared						(1,033)
Net foreign currency translation adjustment, net of tax	(310)						(310)
Net unrealized gains (losses) on investments, net of tax	81,070						81,070
Net actuarial gains (losses)	(178)						(178)
Balance, end of period at Jun. 30, 2016		$ 2,805,367	232	(893,176)	2,781,136	855,070	62,105
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of the accounting standard update for share-based payment transactions					0	0
Balance, beginning of period at Dec. 31, 2016	2,872,286		232	(893,332)	2,779,891	997,890	(12,395)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact to common stock related to extinguishment of Convertible Senior Notes			0
Issuance of common stock under incentive and benefit plans			1
Stock Repurchased and Retired During Period, Value			0		(6)
Share-based compensation					7,676
Issuance of common stock under incentive and benefit plans					3,840
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)					(52,352)
Equity Impact of Termination of Capped Call Transaction					4,083
Change in equity component of currently redeemable convertible senior notes					(16)
Repurchases of common stock under incentive plans				(199)
Net income (loss)	49,130					49,130
Dividends declared						(1,076)
Net foreign currency translation adjustment, net of tax	108						108
Net unrealized gains (losses) on investments, net of tax	30,404						30,404
Net actuarial gains (losses)	0						0
Balance, end of period at Jun. 30, 2017	$ 2,914,144	$ 2,914,144	$ 233	$ (893,531)	2,743,872	1,045,453	$ 18,117
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of the accounting standard update for share-based payment transactions					$ 756	$ (491)