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]
Issuance of common stock under incentive and benefit plans			0
Share-based compensation					4,612
Issuance of common stock under incentive and benefit plans					659
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)					151,639
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)			17
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				0
Net income	$ 66,249					66,249
Dividends declared						(789)
Net foreign currency translation adjustment, net of tax	(85)						(85)
Net unrealized gains (losses) on investments, net of tax	41,531						41,531
Net actuarial gains (losses)	(178)						(178)
Balance, end of period at Mar. 31, 2016		$ 2,660,398	232	(893,176)	2,773,349	757,202	22,791
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]
Issuance of common stock under incentive and benefit plans			1
Share-based compensation					3,222
Issuance of common stock under incentive and benefit plans					3,548
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)					(42,940)
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)			0
Change in equity component of currently redeemable convertible senior notes					(883)
Shares repurchased under share repurchase program (Note 13)			0		0
Repurchases of common stock under incentive plans				(40)
Net income	76,472					76,472
Dividends declared						(538)
Net foreign currency translation adjustment, net of tax	34						34
Net unrealized gains (losses) on investments, net of tax	8,998						8,998
Net actuarial gains (losses)	0						0
Balance, end of period at Mar. 31, 2017	$ 2,920,425	$ 2,920,425	$ 233	$ (893,372)	2,743,594	1,073,333	$ (3,363)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of the accounting standard update for share-based payment transactions					$ 756	$ (491)