Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash equivalents
Total	$ 277,444	$ 269,232	$ 270,811	$ 210,799
Assets measured on a recurring basis
Cash equivalents
Total	1,501	17,996
Short-term investments
Total	66,787	116,050
Assets measured on a recurring basis | U.S. treasuries
Cash equivalents
Total		9,999
Short-term investments
Total	40,008	94,918
Assets measured on a recurring basis | Government agency securities
Cash equivalents
Total		4,998
Short-term investments
Total	10,012	12,988
Assets measured on a recurring basis | Corporate debt
Cash equivalents
Total	1,501
Short-term investments
Total	13,773	8,144
Assets measured on a recurring basis | Commercial paper
Cash equivalents
Total		2,999
Short-term investments
Total	2,994
Assets measured on a recurring basis | Level 1
Cash equivalents
Total		9,999
Short-term investments
Total	40,008	94,918
Assets measured on a recurring basis | Level 1 | U.S. treasuries
Cash equivalents
Total		9,999
Short-term investments
Total	40,008	94,918
Assets measured on a recurring basis | Level 2
Cash equivalents
Total	1,501	7,997
Short-term investments
Total	26,779	21,132
Assets measured on a recurring basis | Level 2 | Government agency securities
Cash equivalents
Total		4,998
Short-term investments
Total	10,012	12,988
Assets measured on a recurring basis | Level 2 | Corporate debt
Cash equivalents
Total	1,501
Short-term investments
Total	13,773	8,144
Assets measured on a recurring basis | Level 2 | Commercial paper
Cash equivalents
Total		$ 2,999
Short-term investments
Total	$ 2,994