Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income tax expense (benefit)	$ 32.3	$ 44.2	$ 42.9
Accumulated other comprehensive loss at beginning of period	876.6
Accumulated other comprehensive loss at end of period	942.4	876.6
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period	(0.1)	(0.1)
Accumulated other comprehensive loss at end of period	0.0	(0.1)	$ (0.1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss	0.1	0.0
Net current period other comprehensive income (loss)	0.1	0.0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income tax expense (benefit)	$ 0.0	$ 0.0