Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 10,508.3	$ 10,216.2	$ 9,770.4
Net Change	259.0	(14.1)	(44.3)
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income		0.0
Balance, end of period	11,091.0	10,508.3	10,216.2
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(453.7)	(414.3)	(370.0)
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income		(25.3)
Balance, end of period	(194.7)	(453.7)	(414.3)
Net Unrealized (Losses) Gains on Debt Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(114.9)	(74.8)	(32.4)
Net Change	228.9	(22.3)	(42.4)
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income		(17.8)
Balance, end of period	114.0	(114.9)	(74.8)
NET UNREALIZED (LOSSES) GAINS ON CASH FLOW HEDGES
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	4.0	4.5	6.1
Net Change	(7.7)	(1.4)	(1.6)
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income		0.9
Balance, end of period	(3.7)	4.0	4.5
NET FOREIGN CURRENCY ADJUSTMENTS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	67.9	(1.8)	(18.5)
Net Change	49.9	22.2	16.7
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income		47.5
Balance, end of period	117.8	67.9	(1.8)
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(410.7)	(342.2)	(325.2)
Net Change	(12.1)	(12.6)	(17.0)
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income		(55.9)
Balance, end of period	$ (422.8)	$ (410.7)	$ (342.2)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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Amount of increase (decrease) in accumulated other comprehensive income (AOCI) for reclassification to retained earnings of tax effect from remeasurement of deferred tax pursuant to Tax Cuts and Jobs Act.

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