Stock-Based Compensation - Employee Stock Purchase Plan (Details) - USD ($) $ in Thousands		3 Months Ended
	Jan. 01, 2017	Mar. 31, 2018	Mar. 31, 2017
Employee Stock Purchase Plan
Stock-based compensation expense		$ 26,128	$ 18,401
Employee stock purchase plan
Employee Stock Purchase Plan
Shares reserved for issuance under ESPP		275,000
Number of shares remaining available for grant		151,859
Stock-based compensation expense	$ 500	$ 400