Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 7,958.8
Other comprehensive income (loss) before reclassifications	12.0	$ 3.1	$ 18.2
Amounts reclassified from accumulated other comprehensive income (loss)	(3.8)	5.3	26.5
Other comprehensive income, net of tax	8.2	8.4	44.7
Ending balance	7,927.1	7,958.8
Accumulated Other Comprehensive Income (Loss), Net of Tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	22.6	14.2	(30.5)
Ending balance	30.8	22.6	14.2
Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	1.4	(3.3)	(41.6)
Other comprehensive income (loss) before reclassifications	18.5	(0.6)	11.8
Amounts reclassified from accumulated other comprehensive income (loss)	(3.8)	5.3	26.5
Other comprehensive income, net of tax	14.7	4.7	38.3
Ending balance	16.1	1.4	(3.3)
Defined Benefit Pension Plan
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	21.2	17.5	11.1
Other comprehensive income (loss) before reclassifications	(6.5)	3.7	6.4
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0	0.0
Other comprehensive income, net of tax	(6.5)	3.7	6.4
Ending balance	$ 14.7	$ 21.2	$ 17.5