Fair Value on a Nonrecurring Basis (Details) $ in Millions	Dec. 31, 2017 USD ($)
Fair value assets and liabilities measured on nonrecurring basis
Long-term debt, fair value	$ 1,342.6
Level 1
Fair value assets and liabilities measured on nonrecurring basis
Long-term debt, fair value	0.0
Level 2
Fair value assets and liabilities measured on nonrecurring basis
Long-term debt, fair value	1,296.4
Level 3
Fair value assets and liabilities measured on nonrecurring basis
Long-term debt, fair value	$ 46.2