Derivatives and Hedging (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018	Jul. 31, 2016	Jul. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Other comprehensive income	$ 185.8	$ 70.8	$ 185.8	$ 70.8	$ 152.6	$ 114.5	$ 21.8	$ 134.2
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Loss amount reclassified within twelve months			3.3
Valuation and amortization of derivative securities
Derivative Instruments, Gain (Loss) [Line Items]
Other comprehensive income	(18.6)	(20.9)	(18.6)	(20.9)	(19.2)	(20.3)	(21.5)	(22.7)
Accumulated other comprehensive income (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Other comprehensive income	(16.2)	(20.0)	(16.2)	(20.0)	$ (18.2)	$ (17.5)	$ (19.8)	$ (20.9)
Accumulated other comprehensive income (loss) | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense	$ 0.8	$ 0.7	$ 2.3	$ 2.2
Treasury Lock | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount										$ 300.0
Fair value of derivative									$ 34.4