Fair Value Measurements - Losses (Gains) on Interest Rate Swaps (Details) - Interest rate swap - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gains recorded in accumulated other comprehensive loss, net of tax	$ (4.2)	$ (3.0)	$ (1.6)
Reclassifications of losses out of accumulated other comprehensive loss	7.3	8.2	5.2
Interest Expense
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Ineffectiveness recorded in interest expense	$ (0.3)	$ 0.0	$ 0.0