Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Cash and cash equivalents	$ 446,938	$ 258,567
Cash
Assets
Cash and cash equivalents	110,853	77,443
Money market funds
Assets
Cash and cash equivalents	315,110	173,631
Domestic corporate commercial paper
Assets
Cash and cash equivalents	13,975	5,487
Domestic corporate debt securities
Assets
Cash and cash equivalents	7,000	2,006
Recurring basis
Assets
Cash and cash equivalents	446,938	258,567
Marketable securities	21,144	73,880
Recurring basis | Domestic corporate debt securities
Assets
Marketable securities	11,655	19,315
Recurring basis | Domestic corporate commercial paper
Assets
Marketable securities	9,489	31,930
Recurring basis | Asset-backed securities
Assets
Marketable securities		22,635
Recurring basis | Cash
Assets
Cash and cash equivalents	110,853	77,443
Recurring basis | Money market funds
Assets
Cash and cash equivalents	315,110	173,631
Recurring basis | Domestic corporate commercial paper
Assets
Cash and cash equivalents	13,975	5,487
Recurring basis | Domestic corporate debt securities
Assets
Cash and cash equivalents	7,000	2,006
Recurring basis | Level 1
Assets
Cash and cash equivalents	425,963	251,074
Marketable securities	0	0
Recurring basis | Level 1 | Domestic corporate debt securities
Assets
Marketable securities	0	0
Recurring basis | Level 1 | Domestic corporate commercial paper
Assets
Marketable securities	0	0
Recurring basis | Level 1 | Asset-backed securities
Assets
Marketable securities		0
Recurring basis | Level 1 | Cash
Assets
Cash and cash equivalents	110,853	77,443
Recurring basis | Level 1 | Money market funds
Assets
Cash and cash equivalents	315,110	173,631
Recurring basis | Level 1 | Domestic corporate commercial paper
Assets
Cash and cash equivalents	0	0
Recurring basis | Level 1 | Domestic corporate debt securities
Assets
Cash and cash equivalents	0	0
Recurring basis | Level 2
Assets
Cash and cash equivalents	20,975	7,493
Marketable securities	21,144	73,880
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Marketable securities	11,655	19,315
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Marketable securities	9,489	31,930
Recurring basis | Level 2 | Asset-backed securities
Assets
Marketable securities		22,635
Recurring basis | Level 2 | Cash
Assets
Cash and cash equivalents	0	0
Recurring basis | Level 2 | Money market funds
Assets
Cash and cash equivalents	0	0
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Cash and cash equivalents	13,975	5,487
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Cash and cash equivalents	7,000	2,006
Recurring basis | Level 3
Assets
Cash and cash equivalents	0	0
Marketable securities	0	0
Recurring basis | Level 3 | Domestic corporate debt securities
Assets
Marketable securities	0	0
Recurring basis | Level 3 | Domestic corporate commercial paper
Assets
Marketable securities	0	0
Recurring basis | Level 3 | Asset-backed securities
Assets
Marketable securities		0
Recurring basis | Level 3 | Cash
Assets
Cash and cash equivalents	0	0
Recurring basis | Level 3 | Money market funds
Assets
Cash and cash equivalents	0	0
Recurring basis | Level 3 | Domestic corporate commercial paper
Assets
Cash and cash equivalents	0	0
Recurring basis | Level 3 | Domestic corporate debt securities
Assets
Cash and cash equivalents	$ 0	$ 0