Convertible Senior Notes Due 2023 (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Notes payable
Total cost of the convertible note hedge transactions			$ 92,897
Strike price of the Warrant Transactions (in dollars per share)	$ 80.9063		$ 80.9063
Proceeds from sale of warrants			$ 65,688
Debt financing costs			$ 10,435
Liability component of the 2023 Notes
December 31, 2018 carrying value		$ 333,310		$ 329,462
2023 Notes
Notes payable
Aggregate principal amount		$ 402,500		402,500
Interest rate (as a percent)	0.625%		0.625%
Conversion rate for the Notes (in shares)		16.8545
Conversion ratio, principal amount		$ 1,000
Conversion price, per share of Common Stock	$ 59.33	$ 59.33	$ 59.33
Convertible note hedge options issued (in shares)	402,500
Shares of common stock issued upon conversion of Notes	6,800,000
Warrants issued	6,783,939		6,783,939
Number of shares per warrant entitled to holder	1
Liability component of the 2023 Notes
Principal amount of the 2023 Notes		$ 402,500		402,500
Debt discount		(76,434)		(76,434)
Deferred financing costs		(8,452)		(8,452)
Accretion of debt discount and deferred financing costs		15,696		11,848
December 31, 2018 carrying value		333,310		$ 329,462
Conversion of debt to equity		$ 0