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
Beginning Balance, at Dec. 31, 2014		$ 2,097,060	$ 209	$ (892,961)	$ 2,531,513	$ 406,814	$ 51,485
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 92,257	92,257				92,257
Net foreign currency translation adjustment, net of tax	(126)	(126)					(126)
Net unrealized gain on investments, net of tax		8,315					8,315
Repurchases of common stock under incentive plans		(188)		(188)
Issuance of common stock under incentive and benefit plans		1,241			1,241
Net actuarial loss	0
Stock-based compensation		3,123			3,123
Change in equity component of currently redeemable convertible senior notes		5,708			5,708
Dividends declared		(478)				(478)
Ending Balance, at Mar. 31, 2015		2,206,912	209	(893,149)	2,541,585	498,593	59,674
Beginning Balance, at Dec. 31, 2015	2,496,931	2,496,931	224	(893,176)	2,716,618	691,742	(18,477)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	66,249	66,249				66,249
Net foreign currency translation adjustment, net of tax	(85)	(85)					(85)
Net unrealized gain on investments, net of tax		41,531					41,531
Issuance of common stock under incentive and benefit plans		659			659
Net actuarial loss	(178)	(178)					(178)
Stock-based compensation		4,612			4,612
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 10)		151,656	17		151,639
Shares repurchased under share repurchase program (Note 16)		(100,188)	(9)		(100,179)
Dividends declared		(789)				(789)
Ending Balance, at Mar. 31, 2016	$ 2,660,398	$ 2,660,398	$ 232	$ (893,176)	$ 2,773,349	$ 757,202	$ 22,791