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]
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)			28		349,033
Issuance of common stock under incentive and benefit plans			1		1,594
Termination of capped calls (Note 11)			(3)		11,976
Shares repurchased under share repurchase program (Note 13)			(9)		(201,991)
Repurchases of common stock under incentive plans				(215)
Stock-based compensation					6,494
Change in equity component of currently redeemable convertible senior notes					11,102
Net income	$ 142,305					142,305
Dividends declared						(958)
Net foreign currency translation adjustment, net of tax	32						32
Net unrealized gains (losses) on investments, net of tax							(63,051)
Net actuarial gains (losses)	0						0
Balance, end of period at Jun. 30, 2015		$ 2,353,398	226	(893,176)	2,709,721	548,161	(11,534)
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]
Impact of extinguishment of Convertible Senior Notes due 2017 and 2019 (Note 11)			17		149,543
Issuance of common stock under incentive and benefit plans			0		726
Termination of capped calls (Note 11)			0		0
Shares repurchased under share repurchase program (Note 13)			(9)		(100,179)
Repurchases of common stock under incentive plans				0
Stock-based compensation					14,428
Change in equity component of currently redeemable convertible senior notes					0
Net income	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
Net actuarial gains (losses)	(178)						(178)
Balance, end of period at Jun. 30, 2016	$ 2,805,367	$ 2,805,367	$ 232	$ (893,176)	$ 2,781,136	$ 855,070	$ 62,105