Fair Value Measurements Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
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	3,717.0	3,605.6
Level 3
Fair value assets and liabilities measured on nonrecurring basis
Total long-term debt	40.3	38.0
Fair Value, Total [Member]
Fair value assets and liabilities measured on nonrecurring basis
Total long-term debt	$ 3,757.3	$ 3,643.6