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 (Loss)	ASU 2016-09 Additional Paid-in Capital	ASU 2016-09 Retained Earnings	ASU 2016-01 Retained Earnings	ASU 2016-01 Accumulated Other Comprehensive Income (Loss)	ASU 2018-02 Retained Earnings	ASU 2018-02 Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard update								$ 756	$ (491)	$ 0	$ 0	$ 0	$ 0
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
Shares repurchased under share repurchase program (Note 14)			0		(6)
Repurchases of common stock under incentive plans				(422)
Issuance of common stock under incentive and benefit plans					4,761
Share-based compensation					10,290
Impact of extinguishment of convertible senior notes					(52,408)
Termination of capped calls					4,109
Net income	$ 114,272					114,272
Dividends declared						(1,614)
Net unrealized gains (losses) on investments, net of tax	36,532						36,532
Net foreign currency translation adjustment, net of tax	136						136
Balance, end of period at Sep. 30, 2017		$ 2,988,202	233	(893,754)	2,747,393	1,110,057	24,273
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard update								$ 0	$ 0	$ 2,061	$ 224	$ (2,724)	$ 2,724
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
Shares repurchased under share repurchase program (Note 14)			(3)		(50,050)
Repurchases of common stock under incentive plans				(747)
Issuance of common stock under incentive and benefit plans					2,593
Share-based compensation					13,808
Impact of extinguishment of convertible senior notes					0
Termination of capped calls					0
Net income	466,232					466,232
Dividends declared						(1,606)
Net unrealized gains (losses) on investments, net of tax	(85,276)						(85,276)
Net foreign currency translation adjustment, net of tax	3						3
Balance, end of period at Sep. 30, 2018	$ 3,347,278	$ 3,347,278	$ 231	$ (894,635)	$ 2,720,626	$ 1,580,296	$ (59,240)