Cash Equivalents and Investments - Tabular Disclosure (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Cash Equivalents and Investments
Cash equivalents, Amortized Cost	$ 117,751	$ 400,304
Cash equivalents	$ 117,751	$ 400,304
Average Maturity	282 days	348 days
Available-for-sale, Amortized Cost	$ 499,281	$ 273,321
Available-for-sale, Unrealized Gain	7
Available-for-sale, Unrealized Losses	(388)	(269)
Investments, available-for-sale	$ 498,900	$ 273,052
Investments, available-for-sale, type	us-gaap:USTreasurySecuritiesMember	us-gaap:USTreasurySecuritiesMember
Total, Amortized Cost	$ 617,032	$ 673,625
Total, Unrealized Gain	7
Total, Unrealized Losses	(388)	(269)
Total, Fair Value	$ 616,651	$ 673,356