Operating Leases - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leased Assets [Line Items]
Total rent expense	$ 2.6	$ 2.7	$ 1.0
Minimum
Operating Leased Assets [Line Items]
Lease expiration date	Dec. 31, 2018
Maximum
Operating Leased Assets [Line Items]
Lease expiration date	Dec. 31, 2027