Mergers and Acquisitions - Purchase Price Allocation (Details) - USD ($) $ in Millions	Dec. 31, 2017	Oct. 02, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets acquired
Prepaid expenses and other current assets	$ 7.6		$ 3.9
Property, plant and equipment	1,129.0		1,085.5
Goodwill	151.0		14.3	$ 14.3
Intangible assets	132.3		0.0
Other noncurrent assets	20.4		18.8
Liabilities assumed
Accrued expenses and other current liabilities	$ 37.2		$ 31.9
OnX Holdings LLC [Member]
Assets acquired
Cash		$ 6.5
Receivables		69.9
Prepaid expenses and other current assets		11.8
Property, plant and equipment		11.6
Goodwill		132.4
Intangible assets		134.0
Other noncurrent assets		3.2
Total assets acquired		369.4
Liabilities assumed
Accounts payable		63.6
Current portion of long-term debt		1.3
Accrued expenses and other current liabilities		18.3
Deferred income tax liabilities		42.2
Long-term debt, less current portion		76.7
Other noncurrent liabilities		1.1
Total liabilities assumed		203.2
Net assets acquired		$ 166.2