Mergers and Acquisitions - Purchase Price Allocation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Oct. 02, 2017
Business Combinations [Abstract]
Goodwill, Purchase Accounting Adjustments	$ 0.2
Purchase Price Allocation Adjustment, Tax Related	0.1
Purchase Price Allocation Adjustment, Working Capital Related	0.4
Assets acquired
Cash			$ 6.5
Receivables			69.9
Prepaid expenses and other current assets	9.4	$ 7.6	11.8
Property, plant and equipment	1,118.8	1,129.0	11.6
Goodwill	150.7	151.0	133.1
Intangible assets	128.7	132.3	134.0
Other noncurrent assets	52.0	52.7	3.2
Total assets acquired			370.1
Liabilities assumed
Accounts payable			63.6
Current portion of long-term debt			1.3
Accrued expenses and other current liabilities	$ 29.2	$ 37.2	18.3
Deferred income tax liabilities			42.3
Long-term debt, less current portion			76.7
Other noncurrent liabilities			1.5
Total liabilities assumed			203.7
Net assets acquired			$ 166.4