Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets
Cash	$ 198,565	$ 159,678
Cash
Assets
Cash	2,270	2,934
Money market funds
Assets
Cash	196,295	83,257
Domestic corporate commercial paper
Assets
Cash	0	39,984
Government-sponsored enterprise debt securities
Assets
Cash		15,996
Domestic corporate debt securities
Assets
Cash		10,007
Asset-backed securities
Assets
Cash		7,500
Recurring basis
Assets
Cash	198,565	159,678
Marketable securities	171,267	313,661
Recurring basis | Domestic corporate debt securities
Assets
Marketable securities	48,394	173,035
Recurring basis | Domestic corporate commercial paper
Assets
Marketable securities	51,179	84,158
Recurring basis | Asset-backed securities
Assets
Marketable securities	71,694	56,468
Recurring basis | Cash
Assets
Cash	2,270	2,934
Recurring basis | Money market funds
Assets
Cash	196,295	83,257
Recurring basis | Domestic corporate commercial paper
Assets
Cash	0	39,984
Recurring basis | Government-sponsored enterprise debt securities
Assets
Cash		15,996
Recurring basis | Domestic corporate debt securities
Assets
Cash		10,007
Recurring basis | Asset-backed securities
Assets
Cash		7,500
Recurring basis | Level 1
Assets
Cash	198,565	86,191
Recurring basis | Level 1 | Cash
Assets
Cash	2,270	2,934
Recurring basis | Level 1 | Money market funds
Assets
Cash	196,295	83,257
Recurring basis | Level 2
Assets
Cash	0	73,487
Marketable securities	171,267	313,661
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Marketable securities	48,394	173,035
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Marketable securities	51,179	84,158
Recurring basis | Level 2 | Asset-backed securities
Assets
Marketable securities	71,694	56,468
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Cash	$ 0	39,984
Recurring basis | Level 2 | Government-sponsored enterprise debt securities
Assets
Cash		15,996
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Cash		10,007
Recurring basis | Level 2 | Asset-backed securities
Assets
Cash		$ 7,500