Changes in Accumulated Other Comprehensive Income by Component - Summary of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income as of December 31, 2018	$ 7,927.1
Other comprehensive income before reclassifications	(11.3)
Amounts reclassified from accumulated other comprehensive income	(0.1)	$ (0.2)
Other comprehensive income, net of tax	(11.4)	$ 19.1
Adoption of accounting standards	5.4
Accumulated other comprehensive income as of March 31, 2019	7,919.5
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income as of December 31, 2018	30.8
Adoption of accounting standards	5.4
Accumulated other comprehensive income as of March 31, 2019	24.8
Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income as of December 31, 2018	16.1
Other comprehensive income before reclassifications	(11.3)
Amounts reclassified from accumulated other comprehensive income	(0.1)
Other comprehensive income, net of tax	(11.4)
Adoption of accounting standards	5.4
Accumulated other comprehensive income as of March 31, 2019	10.1
Defined Benefit Pension Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income as of December 31, 2018	14.7
Other comprehensive income before reclassifications	0.0
Amounts reclassified from accumulated other comprehensive income	0.0
Other comprehensive income, net of tax	0.0
Adoption of accounting standards	0.0
Accumulated other comprehensive income as of March 31, 2019	$ 14.7