Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Recurring basis
Liabilities:
Liabilities measured and recorded at fair value	$ 0	$ 0
Level 1 | Recurring basis
Assets:
Total	1,068,619	642,644
U.S. Treasury Securities
Assets:
Available-for-sale securities	903,415	561,327
U.S. Treasury Securities | Level 1 | Recurring basis
Assets:
Available-for-sale securities	903,415	561,327
Cash equivalents	9,988
U.S. Agency Securities
Assets:
Available-for-sale securities	17,621	17,663
U.S. Agency Securities | Level 1 | Recurring basis
Assets:
Available-for-sale securities	17,621	17,663
Time Deposits
Assets:
Time deposits	10,172	18,924
Time Deposits | Level 1 | Recurring basis
Assets:
Time deposits	10,172	18,924
Money market funds | Level 1 | Recurring basis
Assets:
Cash equivalents	$ 127,423	$ 44,730