Fair Value Measurements Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Level 1
Fair value assets and liabilities measured on nonrecurring basis
Total long-term debt	$ 0.0	$ 0.0
Level 2
Fair value assets and liabilities measured on nonrecurring basis
Total long-term debt	1,261.9	1,296.4
Level 3
Fair value assets and liabilities measured on nonrecurring basis
Total long-term debt	48.4	46.2
Fair Value, Total [Member]
Fair value assets and liabilities measured on nonrecurring basis
Total long-term debt	$ 1,310.3	$ 1,342.6