Assets - Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Cash equivalents
Total	$ 423,661	$ 277,444	$ 243,722	$ 269,232
Short-term investments
Total	258,196	66,787
Transfer of assets between levels	0
Assets measured on a recurring basis
Cash equivalents
Total	101,354	1,501
Short-term investments
Total	258,196	66,787
Assets measured on a recurring basis | U.S. treasuries
Cash equivalents
Total	79,962
Short-term investments
Total	180,013	40,008
Assets measured on a recurring basis | Government agency securities
Cash equivalents
Total	9,997
Short-term investments
Total	29,998	10,012
Assets measured on a recurring basis | Corporate debt
Cash equivalents
Total	2,401	1,501
Short-term investments
Total	30,242	13,773
Assets measured on a recurring basis | Commercial paper
Cash equivalents
Total	8,994
Short-term investments
Total	17,943	2,994
Assets measured on a recurring basis | Level 1
Cash equivalents
Total	79,962
Short-term investments
Total	180,013	40,008
Assets measured on a recurring basis | Level 1 | U.S. treasuries
Cash equivalents
Total	79,962
Short-term investments
Total	180,013	40,008
Assets measured on a recurring basis | Level 2
Cash equivalents
Total	21,392	1,501
Short-term investments
Total	78,183	26,779
Assets measured on a recurring basis | Level 2 | Government agency securities
Cash equivalents
Total	9,997
Short-term investments
Total	29,998	10,012
Assets measured on a recurring basis | Level 2 | Corporate debt
Cash equivalents
Total	2,401	1,501
Short-term investments
Total	30,242	13,773
Assets measured on a recurring basis | Level 2 | Commercial paper
Cash equivalents
Total	8,994
Short-term investments
Total	$ 17,943	$ 2,994