Accumulated Other Comprehensive Income (Loss) (Reclassification Adjustment Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment Security Gains (Losses), net	$ (0.2)	$ (0.3)
Realized Gains on Cash Flow Hedges	34.8	39.7
Net Unrealized Gains (Losses) on Debt Securities Available for Sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pension and Other Postretirement Benefit Adjustments	0.0	0.2
Net Unrealized Gains (Losses) on Debt Securities Available for Sale | Net Gain/(Loss) Reclassified from AOCI to Net Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment Security Gains (Losses), net	0.0
Net Unrealized Gains (Losses) on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pension and Other Postretirement Benefit Adjustments	(16.6)	(5.1)
Net Unrealized Gains (Losses) on Cash Flow Hedges | Net Gain/(Loss) Reclassified from AOCI to Net Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized Gains on Cash Flow Hedges	(16.6)
Amortization of Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pension and Other Postretirement Benefit Adjustments	9.2
Amortization of Prior Service Cost
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pension and Other Postretirement Benefit Adjustments	(0.1)
Net Pension and Other Postretirement Benefit Adjustments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pension and Other Postretirement Benefit Adjustments	$ 9.1	$ 6.4