Commitments and Contingencies - Lease Commitments (Details) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Dec. 31, 2017	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) renewal_option	Sep. 30, 2017 USD ($)
Operating Leased Assets [Line Items]
Operating leases, rent expense		$ 1.5	$ 1.1	$ 4.1	$ 2.9
Transportation Equipment
Operating Leased Assets [Line Items]
Operating leases, renewal term	6 years
Operating leases, rent expense		$ 0.5		$ 1.1
Operating lease term	2 years
Minimum
Operating Leased Assets [Line Items]
Operating lease, number of renewal options (or more) | renewal_option				1
Operating leases, renewal term				2 years
Maximum
Operating Leased Assets [Line Items]
Operating leases, renewal term				20 years