Investments and Fair Value Measurements - Maturity of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due less than one year	$ 193,309	$ 157,651
Due in one to five years	87,616	119,224
Total	280,925	276,875
Fair Value
Due less than one year	192,753	157,573
Due in one to five years	87,065	118,841
Total	$ 279,818	$ 276,414