Reclassification Adjustments out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment Security Losses, net	$ (3.2)	$ (0.3)	$ (4.3)
Other Operating Income	241.2	247.1	153.5
Net Unrealized (Losses) Gains on Securities Available for Sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	(0.5)	0.3	0.1
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	(0.5)	0.3	0.1
Net Unrealized Gains (Losses) on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	(1.9)	4.7	(4.0)
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	(1.9)	4.7	(4.0)
Amortization of Net Actuarial Losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	25.6	38.5	25.1
Amortization of Prior Service Cost
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	(0.2)	(0.2)	0.1
Net Pension and Other Postretirement Benefit Adjustments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	$ 25.4	$ 38.3	$ 25.2