Accumulated Other Comprehensive Loss (Components Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	$ (64.6)	$ (55.9)	$ 48.4
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.5	0.5	(32.7)
Net current-period other comprehensive income (loss)	(64.1)	(55.4)	15.7
Accumulated other comprehensive loss	(156.7)	(92.6)	(37.2)	$ (43.4)
Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss			(9.5)
Gains and Losses on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	3.3	6.1	(3.0)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.6)	(0.4)	0.4
Net current-period other comprehensive income (loss)	2.7	5.7	(2.6)
Accumulated other comprehensive loss	6.0	3.3	(2.4)	0.2
Gains and Losses on Securities | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss			0.0
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(70.8)	(64.3)	55.8
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.1	0.1	(33.1)
Net current-period other comprehensive income (loss)	(70.7)	(64.2)	22.7
Accumulated other comprehensive loss	(147.9)	(77.2)	(13.0)	(29.4)
Gains and Losses on Cash Flow Hedges | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss			(6.3)
Pension and OPEB Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	2.9	2.3	(4.4)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1.0	0.8	0.0
Net current-period other comprehensive income (loss)	3.9	3.1	(4.4)
Accumulated other comprehensive loss	$ (14.8)	$ (18.7)	(21.8)	$ (14.2)
Pension and OPEB Items | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss			$ (3.2)