Commitments and Contingencies, Rollforward of CARO Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Asset Retirement Obligation, Roll Forward
Beginning balance	$ 4.9	$ 4.8	$ 4.4
Liabilities incurred during the period	0.0	0.0	0.0
Liabilities settled during the period	(0.1)	(0.2)	0.0
Accretion expense	0.5	0.3	0.4
Adjustment to accretion expense due to revisions to estimated cash flow and timing of expenditure1	0.2	0.0	0.0
Ending balance	$ 5.5	$ 4.9	$ 4.8