Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Amortizing	$ 25.2	$ 20.9
Non-amortizing	$ 6.0	13.0
Weighted average remaining life for amortizing customer-related assets and all amortizing intangibles (in years)	13 years
Residual values	$ 0.0
Amortization expense	2.6	2.5	$ 3.1
Amortization expense - 2018	2.6
Amortization expense - 2019	2.6
Amortization expense - 2020	2.6
Amortization expense - 2021	2.6
Amortization expense - 2022	2.6
Customer-related assets
Finite-Lived Intangible Assets [Line Items]
Amortizing	18.6	20.9
Reclassification of market-related assets from non-amortizing to amortizing	7.0
Accumulated amortization	14.6	12.0
Market-related assets
Finite-Lived Intangible Assets [Line Items]
Amortizing	6.6	0.0
Accumulated amortization	4.3	4.5
Customer-related assets
Finite-Lived Intangible Assets [Line Items]
Non-amortizing	0.0	0.0
Market-related assets
Finite-Lived Intangible Assets [Line Items]
Non-amortizing	$ 6.0	$ 13.0