Intangible Assets - Schedule of Finite-Lived Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net	$ 8.2	$ 6.8
Capacity Agreement
Finite-Lived Intangible Assets [Line Items]
Cost	10.1	10.1
Accumulated amortization	(1.9)	(3.3)
Impairment charges	0.0
Intangible assets, net	$ 8.2	$ 6.8