Accumulated Other Comprehensive Income (Loss) - Details of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
BEFORE TAX
Net Change	$ 347.9	$ 11.6	$ (156.7)
TAX EFFECT
Net Change	(88.9)	(25.7)	112.4
AFTER TAX
Other comprehensive income (loss) before reclassifications	259.0	(14.1)	(44.3)
Unrealized Gains (Losses) on Debt Securities Available for Sale
BEFORE TAX
Gains (Losses)	306.1	(31.9)	(70.2)
Reclassification Adjustment for Losses (Gains) Included in Net Income	1.1	0.5	1.4
Net Change	307.2	(31.4)	(68.8)
TAX EFFECT
Gains (Losses)	(78.0)	9.2	26.9
Reclassification Adjustment for Losses (Gains) Included in Net Income	(0.3)	(0.1)	(0.5)
Net Change	(78.3)	9.1	26.4
AFTER TAX
Gains (Losses)	228.1	(22.7)	(43.3)
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.8	0.4	0.9
Other comprehensive income (loss) before reclassifications	228.9	(22.3)	(42.4)
Unrealized (Losses) Gains on Cash Flow Hedges
BEFORE TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(26.7)	(71.1)	(24.5)
Net Change	(10.3)	(1.8)	9.3
TAX EFFECT
Reclassification Adjustment for Losses (Gains) Included in Net Income	6.6	17.7	9.4
Net Change	2.6	0.4	(10.9)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(20.1)	(53.4)	(15.1)
Other comprehensive income (loss) before reclassifications	(7.7)	(1.4)	(1.6)
Foreign Currency Adjustments
BEFORE TAX
Net Change	65.6	63.4	(64.7)
TAX EFFECT
Net Change	(15.7)	(41.2)	81.4
AFTER TAX
Other comprehensive income (loss) before reclassifications	49.9	22.2	16.7
Foreign Currency Translation Adjustments
BEFORE TAX
Net Change	6.4	(107.8)	156.5
TAX EFFECT
Net Change	(1.6)	1.5	(3.1)
AFTER TAX
Other comprehensive income (loss) before reclassifications	4.8	(106.3)	153.4
Long-Term Intra-Entity Foreign Currency Transaction (Losses) Gains
BEFORE TAX
Net Change	(0.5)	(1.8)	2.0
TAX EFFECT
Net Change	0.1	0.5	(0.7)
AFTER TAX
Other comprehensive income (loss) before reclassifications	(0.4)	(1.3)	1.3
Net Investment Hedge Gains (Losses)
BEFORE TAX
Net Change	59.7	173.0	(223.2)
TAX EFFECT
Net Change	(14.2)	(43.2)	85.2
AFTER TAX
Other comprehensive income (loss) before reclassifications	45.5	129.8	(138.0)
Pension and Other Postretirement Benefit Adjustments
BEFORE TAX
Gains (Losses)	(36.8)	(54.9)	(58.4)
Net Change	(14.6)	(18.6)	(32.5)
TAX EFFECT
Gains (Losses)	7.9	9.6	25.4
Net Change	2.5	6.0	15.5
AFTER TAX
Gains (Losses)	(28.9)	(45.3)	(33.0)
Other comprehensive income (loss) before reclassifications	(12.1)	(12.6)	(17.0)
Amortization of Net Actuarial Loss
BEFORE TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	22.4	36.6	26.0
TAX EFFECT
Reclassification Adjustment for Losses (Gains) Included in Net Income	(5.4)	(3.6)	(9.9)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	17.0	33.0	16.1
Amortization of Prior Service Cost
BEFORE TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(0.2)	(0.3)	(0.1)
TAX EFFECT
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	0.0	0.0
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(0.2)	(0.3)	(0.1)
Foreign Exchange Contracts \| Unrealized (Losses) Gains on Cash Flow Hedges
BEFORE TAX
Gains (Losses)	14.9	70.5	32.5
TAX EFFECT
Gains (Losses)	(3.7)	(17.6)	(19.5)
AFTER TAX
Gains (Losses)	11.2	52.9	13.0
Interest Rate Contracts \| Unrealized (Losses) Gains on Cash Flow Hedges
BEFORE TAX
Gains (Losses)	1.5	(1.2)	1.3
TAX EFFECT
Gains (Losses)	(0.3)	0.3	(0.8)
AFTER TAX
Gains (Losses)	$ 1.2	$ (0.9)	$ 0.5

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