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					17,632
Issuance of common stock under incentive and benefit plans					1,711
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)					143,078
Equity Impact of Termination of Capped Call Transaction					0
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						84,318
Net actuarial gains (losses)	(22)						(22)
Balance, end of period at Sep. 30, 2016		$ 2,888,706	232	(893,197)	2,778,860	937,338	65,473
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					10,290
Issuance of common stock under incentive and benefit plans					4,761
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)					(52,408)
Equity Impact of Termination of Capped Call Transaction					4,109
Repurchases of common stock under incentive plans				(422)
Net income	114,272					114,272
Dividends declared						(1,614)
Net foreign currency translation adjustment, net of tax	136						136
Net unrealized gains (losses) on investments, net of tax	36,532						36,532
Net actuarial gains (losses)	0						0
Balance, end of period at Sep. 30, 2017	$ 2,988,202	$ 2,988,202	$ 233	$ (893,754)	2,747,393	1,110,057	$ 24,273
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of the accounting standard update for share-based payment transactions					$ 756	$ (491)