Changes in Accumulated Other Comprehensive Income by Component - Summary of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 7,922.6	$ 7,914.3	$ 7,929.5	$ 7,961.1
Other comprehensive income before reclassifications			(32.0)
Amounts reclassified from accumulated other comprehensive income	(0.2)	(0.8)	(0.3)	(1.0)
Other comprehensive income (loss), net of tax	(20.9)	11.1	(32.3)	30.2
Adoption of accounting standards			5.4
Balance at end of period	7,955.2	7,848.8	7,955.2	7,848.8
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	24.8	41.7	30.8	22.6
Balance at end of period	3.9	$ 52.8	3.9	$ 52.8
Cash Flow Hedge
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			16.1
Other comprehensive income before reclassifications			(31.2)
Amounts reclassified from accumulated other comprehensive income			(0.3)
Other comprehensive income (loss), net of tax			(31.5)
Adoption of accounting standards			5.4
Balance at end of period	(10.0)		(10.0)
Defined Benefit Pension Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			14.7
Other comprehensive income before reclassifications			(0.8)
Amounts reclassified from accumulated other comprehensive income			0.0
Other comprehensive income (loss), net of tax			(0.8)
Adoption of accounting standards			0.0
Balance at end of period	$ 13.9		$ 13.9