Stock-Based Compensation - Employee Stock Purchase Plan (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Employee Stock Purchase Plan
Stock-based compensation expense	$ 28,409	$ 23,508	$ 54,537	$ 41,909
Shares issued under ESPP				17,684
Employee stock purchase plan
Employee Stock Purchase Plan
Shares reserved for issuance under ESPP	550,000		550,000
Number of shares remaining available for grant	426,859		426,859
Stock-based compensation expense			$ 900	$ 1,000