Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Cash and cash equivalents	$ 59,321	$ 118,564
Cash
Assets
Cash and cash equivalents	20,448	73,302
Money market
Assets
Cash and cash equivalents	38,873	325
Domestic corporate commercial paper
Assets
Cash and cash equivalents		44,937
Recurring basis | Estimated Fair Value
Assets
Cash and cash equivalents	59,321	118,564
Marketable securities	177,140	311,692
Recurring basis | Domestic corporate debt securities | Estimated Fair Value
Assets
Marketable securities	132,356	207,086
Recurring basis | Domestic corporate commercial paper | Estimated Fair Value
Assets
Marketable securities		29,837
Recurring basis | Asset-backed securities | Estimated Fair Value
Assets
Marketable securities	44,784	74,769
Recurring basis | Cash | Estimated Fair Value
Assets
Cash and cash equivalents	20,448	73,302
Recurring basis | Money market | Estimated Fair Value
Assets
Cash and cash equivalents	38,873	325
Recurring basis | Domestic corporate commercial paper | Estimated Fair Value
Assets
Cash and cash equivalents		44,937
Recurring basis | Level 1
Assets
Cash and cash equivalents	59,321	73,627
Recurring basis | Level 1 | Cash
Assets
Cash and cash equivalents	20,448	73,302
Recurring basis | Level 1 | Money market
Assets
Cash and cash equivalents	38,873	325
Recurring basis | Level 2
Assets
Cash and cash equivalents	0	44,937
Marketable securities	177,140	311,692
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Marketable securities	132,356	207,086
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Marketable securities		29,837
Recurring basis | Level 2 | Asset-backed securities
Assets
Marketable securities	$ 44,784	74,769
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Cash and cash equivalents		$ 44,937