Accumulated Other Comprehensive Income (Loss) - Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment Security Losses, net	$ (1.6)	$ (3.2)	$ (0.3)
Other Operating Income	157.5	241.2	247.1
Net Unrealized (Losses) Gains on Securities Available for Sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	1.4	(0.5)	0.3
Net Unrealized (Losses) Gains on Securities Available for Sale | Net Gain/(Loss) Reclassified from AOCI to Earnings
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment Security Losses, net	1.4	(0.5)	0.3
Net Unrealized Gains (Losses) on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	(24.5)	(1.9)	4.7
Net Unrealized Gains (Losses) on Cash Flow Hedges | Net Gain/(Loss) Reclassified from AOCI to Earnings
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Operating Income	(24.5)	(1.9)	4.7
Amortization of Net Actuarial Losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	26.0	25.6	38.5
Amortization of Prior Service Cost
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	(0.1)	(0.2)	(0.2)
Net Pension and Other Postretirement Benefit Adjustments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	$ 25.9	$ 25.4	$ 38.3