Accumulated Other Comprehensive Loss (Components Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	[1]	$ (80.8)	$ (20.9)	$ (37.2)	$ (43.4)
Other Comprehensive Income before reclassifications	[1]	(49.6)	22.6	(93.9)	69.3
Amounts reclassified from accumulated other comprehensive loss	[1]	0.1	0.8	0.8	(13.9)
Net current-period other comprehensive income (loss)	[1]	(49.5)	23.4	(93.1)	55.4
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	[1]					$ (9.5)
Ending Balance	[1]	(130.3)	2.5	(130.3)	2.5
Gains and Losses on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	[1]	2.5	(2.2)	(2.4)	0.2
Other Comprehensive Income before reclassifications	[1]	0.9	0.0	5.9	(2.5)
Amounts reclassified from accumulated other comprehensive loss	[1]	(0.2)	0.1	(0.3)	0.2
Net current-period other comprehensive income (loss)	[1]	0.7	0.1	5.6	(2.3)
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	[1]					0.0
Ending Balance	[1]	3.2	(2.1)	3.2	(2.1)
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	[1]	(62.8)	(1.7)	(13.0)	(29.4)
Other Comprehensive Income before reclassifications	[1]	(50.7)	21.6	(100.5)	70.8
Amounts reclassified from accumulated other comprehensive loss	[1]	0.1	0.9	0.1	(14.3)
Net current-period other comprehensive income (loss)	[1]	(50.6)	22.5	(100.4)	56.5
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	[1]					(6.3)
Ending Balance	[1]	(113.4)	20.8	(113.4)	20.8
Pension and OPEB Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	[1]	(20.5)	(17.0)	(21.8)	(14.2)
Other Comprehensive Income before reclassifications	[1]	0.2	1.0	0.7	1.0
Amounts reclassified from accumulated other comprehensive loss	[1]	0.2	(0.2)	1.0	0.2
Net current-period other comprehensive income (loss)	[1]	0.4	0.8	1.7	1.2
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	[1]					$ (3.2)
Ending Balance	[1]	$ (20.1)	$ (16.2)	$ (20.1)	$ (16.2)

[1] All amounts are net of tax. Amounts in parentheses indicate debits.