Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 10,216.2	$ 9,770.4
NET CHANGE	(14.1)	(44.3)	$ 2.7
Balance, end of period	10,508.3	10,216.2	9,770.4
Net Unrealized (Losses) Gains on Debt Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(74.8)	(32.4)	(31.0)
NET CHANGE	(22.3)	(42.4)	(1.4)
RECLASSIFICATION OF CERTAIN TAX EFFECTS FROM AOCI	(17.8)
Balance, end of period	(114.9)	(74.8)	(32.4)
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	4.5	6.1	(3.0)
NET CHANGE	(1.4)	(1.6)	9.1
RECLASSIFICATION OF CERTAIN TAX EFFECTS FROM AOCI	0.9
Balance, end of period	4.0	4.5	6.1
Net Foreign Currency Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(1.8)	(18.5)	(17.6)
NET CHANGE	22.2	16.7	(0.9)
RECLASSIFICATION OF CERTAIN TAX EFFECTS FROM AOCI	47.5
Balance, end of period	67.9	(1.8)	(18.5)
Net Pension and Other Postretirement Benefit Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(342.2)	(325.2)	(321.1)
NET CHANGE	(12.6)	(17.0)	(4.1)
RECLASSIFICATION OF CERTAIN TAX EFFECTS FROM AOCI	(55.9)
Balance, end of period	(410.7)	(342.2)	(325.2)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(414.3)	(370.0)	(372.7)
NET CHANGE	(14.1)	(44.3)	2.7
RECLASSIFICATION OF CERTAIN TAX EFFECTS FROM AOCI	(25.3)
Balance, end of period	$ (453.7)	$ (414.3)	$ (370.0)