Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets
Cash	$ 174,621	$ 258,567
Cash
Assets
Cash	96,823	77,443
Money market funds
Assets
Cash	77,798	173,631
Domestic corporate commercial paper
Assets
Cash		5,487
Domestic corporate debt securities
Assets
Cash		2,006
Recurring basis
Assets
Cash	174,621	258,567
Marketable securities	107,486	73,880
Recurring basis | Domestic corporate debt securities
Assets
Marketable securities	37,431	19,315
Recurring basis | Domestic corporate commercial paper
Assets
Marketable securities	51,907	31,930
Recurring basis | Asset-backed securities
Assets
Marketable securities	18,148	22,635
Recurring basis | Cash
Assets
Cash	96,823	77,443
Recurring basis | Money market funds
Assets
Cash	77,798	173,631
Recurring basis | Domestic corporate commercial paper
Assets
Cash		5,487
Recurring basis | Domestic corporate debt securities
Assets
Cash		2,006
Recurring basis | Level 1
Assets
Cash	174,621	251,074
Recurring basis | Level 1 | Cash
Assets
Cash	96,823	77,443
Recurring basis | Level 1 | Money market funds
Assets
Cash	77,798	173,631
Recurring basis | Level 2
Assets
Cash	0	7,493
Marketable securities	107,486	73,880
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Marketable securities	37,431	19,315
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Marketable securities	51,907	31,930
Recurring basis | Level 2 | Asset-backed securities
Assets
Marketable securities	$ 18,148	22,635
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Cash		5,487
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Cash		$ 2,006