Investments - Schedule of Available-for-Sale Investment Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in less than 1 year	$ 40,973	$ 55,801
Due in 1-5 years	26,028	25,040
Total	67,001	80,841
Fair Value
Due in less than 1 year	40,864	55,735
Due in 1-5 years	25,890	24,913
Total	$ 66,754	$ 80,648