Investments - Gross amortized cost and estimated fair value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Gross Amortized Cost
Due in one year or less	$ 57,264	$ 58,883
Total	57,264	58,883
Fair Value
Due in one year or less	57,254	58,872
Total	$ 57,254	$ 58,872