Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Cash and cash equivalents	$ 118,564	$ 258,567
Cash
Assets
Cash and cash equivalents	73,302	77,443
Money market
Assets
Cash and cash equivalents	325	173,631
Domestic corporate commercial paper
Assets
Cash and cash equivalents	44,937	5,487
Domestic corporate debt securities
Assets
Cash and cash equivalents		2,006
Recurring basis | Estimated Fair Value
Assets
Cash and cash equivalents	118,564	258,567
Marketable securities	311,692	73,880
Recurring basis | Domestic corporate debt securities | Estimated Fair Value
Assets
Marketable securities	207,086	19,315
Recurring basis | Domestic corporate commercial paper | Estimated Fair Value
Assets
Marketable securities	29,837	31,930
Recurring basis | Asset-backed securities | Estimated Fair Value
Assets
Marketable securities	74,769	22,635
Recurring basis | Cash | Estimated Fair Value
Assets
Cash and cash equivalents	73,302	77,443
Recurring basis | Money market | Estimated Fair Value
Assets
Cash and cash equivalents	325	173,631
Recurring basis | Domestic corporate commercial paper | Estimated Fair Value
Assets
Cash and cash equivalents	44,937	5,487
Recurring basis | Domestic corporate debt securities | Estimated Fair Value
Assets
Cash and cash equivalents		2,006
Recurring basis | Level 1
Assets
Cash and cash equivalents	73,627	251,074
Recurring basis | Level 1 | Cash
Assets
Cash and cash equivalents	73,302	77,443
Recurring basis | Level 1 | Money market
Assets
Cash and cash equivalents	325	173,631
Recurring basis | Level 2
Assets
Cash and cash equivalents	44,937	7,493
Marketable securities	311,692	73,880
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Marketable securities	207,086	19,315
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Marketable securities	29,837	31,930
Recurring basis | Level 2 | Asset-backed securities
Assets
Marketable securities	74,769	22,635
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Cash and cash equivalents	$ 44,937	5,487
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Cash and cash equivalents		$ 2,006