Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets
Cash and cash equivalents	$ 135,110	$ 258,567
Cash
Assets
Cash and cash equivalents	98,201	77,443
Money market funds
Assets
Cash and cash equivalents	30,573	173,631
Domestic corporate commercial paper
Assets
Cash and cash equivalents		5,487
Domestic corporate debt securities
Assets
Cash and cash equivalents	6,336	2,006
Recurring basis
Assets
Cash and cash equivalents	135,110	258,567
Marketable securities	79,606	73,880
Recurring basis | Domestic corporate debt securities
Assets
Marketable securities	34,909	19,315
Recurring basis | Domestic corporate commercial paper
Assets
Marketable securities	44,697	31,930
Recurring basis | Asset-backed securities
Assets
Marketable securities		22,635
Recurring basis | Cash
Assets
Cash and cash equivalents	98,201	77,443
Recurring basis | Money market funds
Assets
Cash and cash equivalents	30,573	173,631
Recurring basis | Domestic corporate commercial paper
Assets
Cash and cash equivalents		5,487
Recurring basis | Domestic corporate debt securities
Assets
Cash and cash equivalents	6,336	2,006
Recurring basis | Level 1
Assets
Cash and cash equivalents	128,774	251,074
Recurring basis | Level 1 | Cash
Assets
Cash and cash equivalents	98,201	77,443
Recurring basis | Level 1 | Money market funds
Assets
Cash and cash equivalents	30,573	173,631
Recurring basis | Level 1 | Domestic corporate debt securities
Assets
Cash and cash equivalents	0
Recurring basis | Level 2
Assets
Cash and cash equivalents	6,336	7,493
Marketable securities	79,606	73,880
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Marketable securities	34,909	19,315
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Marketable securities	44,697	31,930
Recurring basis | Level 2 | Asset-backed securities
Assets
Marketable securities		22,635
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Cash and cash equivalents		5,487
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Cash and cash equivalents	$ 6,336	$ 2,006