Investments - Table of Amortized Cost and Fair Value by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 32,077	$ 26,399
Due after one year through five years	136,882	86,287
Due after five years through ten years	34,859	41,442
Due after ten years	9,474	5,065
Asset-backed securities	46,541	23,369
Amortized Cost	259,833	182,562
Fair Value
Due in one year or less	32,007	26,363
Due after one year through five years	135,239	85,852
Due after five years through ten years	33,700	41,085
Due after ten years	9,269	5,067
Asset-backed securities	45,569	23,493
Fair Value	$ 255,784	$ 181,860