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	Accounting Standards Update 2016-01 [Member] Retained Earnings	Accounting Standards Update 2016-01 [Member] Accumulated Other Comprehensive Income	Accounting Standards Update 2018-02 [Member] Retained Earnings	Accounting Standards Update 2018-02 [Member] Accumulated Other Comprehensive Income	Accounting Standards Update 2016-09 [Member] Additional Paid-in Capital	Accounting Standards Update 2016-09 [Member] Retained Earnings
Balance, beginning of period at Dec. 31, 2016			$ 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
Repurchases of common stock under incentive plans				(199)
Issuance of common stock under incentive and benefit plans					3,840
Share-based compensation					7,676
Impact of extinguishment of convertible senior notes					(52,352)
Equity Impact of Termination of Capped Call Transaction					4,083
Change in equity component of currently redeemable convertible senior notes					(16)
Shares repurchased under share repurchase program (Note 14)			0		(6)
Net income	$ 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
Balance, end of period at Jun. 30, 2017		$ 2,914,144	233	(893,531)	2,743,872	1,045,453	18,117
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption								$ 0	$ 0	$ 0	$ 0	$ 756	$ (491)
Balance, beginning of period at Dec. 31, 2017	3,000,038		233	(893,888)	2,754,275	1,116,333	23,085
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1
Repurchases of common stock under incentive plans				(722)
Issuance of common stock under incentive and benefit plans					1,579
Share-based compensation					9,622
Impact of extinguishment of convertible senior notes					0
Equity Impact of Termination of Capped Call Transaction					0
Change in equity component of currently redeemable convertible senior notes					0
Shares repurchased under share repurchase program (Note 14)			(3)		(50,050)
Net income	323,435					323,435
Dividends declared						(1,073)
Net foreign currency translation adjustment, net of tax	3						3
Net unrealized gains (losses) on investments, net of tax	(83,979)						(83,979)
Balance, end of period at Jun. 30, 2018	$ 3,201,136	$ 3,201,136	$ 231	$ (894,610)	$ 2,715,426	$ 1,438,032	$ (57,943)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption								$ 2,061	$ 224	$ (2,724)	$ 2,724	$ 0	$ 0