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				(40)
Issuance of common stock under incentive and benefit plans					3,548
Share-based compensation					3,222
Impact of extinguishment of convertible senior notes					(42,940)
Change in equity component of currently redeemable convertible senior notes					(883)
Shares repurchased under share repurchase program (Note 14)					0
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
Balance, end of period at Mar. 31, 2017		$ 2,920,425	233	(893,372)	2,743,594	1,073,333	(3,363)
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			0
Repurchases of common stock under incentive plans				(303)
Issuance of common stock under incentive and benefit plans					1,433
Share-based compensation					2,528
Impact of extinguishment of convertible senior notes					0
Change in equity component of currently redeemable convertible senior notes					0
Shares repurchased under share repurchase program (Note 14)					(10,003)
Net income	114,486					114,486
Dividends declared						(540)
Net foreign currency translation adjustment, net of tax	3						3
Net unrealized gains (losses) on investments, net of tax	(57,511)						(57,511)
Balance, end of period at Mar. 31, 2018	$ 3,052,416	$ 3,052,416	$ 233	$ (894,191)	$ 2,748,233	$ 1,229,616	$ (31,475)
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