Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	$ 5,573.1
Accumulated other comprehensive (loss) income at end of period	6,081.8	$ 5,573.1
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	(20.2)	(20.9)
Other comprehensive income (loss) before reclassifications	0.0	0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	0.0
Net current period other comprehensive income (loss)	0.8	0.7
Accumulated other comprehensive (loss) income at end of period	(19.4)	(20.2)
Gains and Losses on Cash Flow Hedges | Interest Rate Swap [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.8	0.7
Unrealized Gains and Losses on Marketable Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	0.0	0.1
Other comprehensive income (loss) before reclassifications		(0.1)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0.0
Net current period other comprehensive income (loss)		(0.1)
Accumulated other comprehensive (loss) income at end of period		0.0
Unrealized Gains and Losses on Marketable Securities | Interest Rate Swap [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0.0
Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	(2.9)	(3.7)
Other comprehensive income (loss) before reclassifications	(0.4)	0.6
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.2	0.2
Net current period other comprehensive income (loss)	(0.2)	0.8
Accumulated other comprehensive (loss) income at end of period	(3.1)	(2.9)
Defined Benefit Pension and Postretirement Items | Interest Rate Swap [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	0.0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	(23.1)	(24.5)
Other comprehensive income (loss) before reclassifications	(0.4)	0.5
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.2	0.2
Net current period other comprehensive income (loss)	0.6	1.4
Accumulated other comprehensive (loss) income at end of period	(22.5)	(23.1)
Total | Interest Rate Swap [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ 0.8	$ 0.7