Accumulated Other Comprehensive Income (Loss) - Details of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
BEFORE TAX
Net Change	$ 790.8	$ 347.9	$ 11.6
TAX EFFECT
Net Change	(168.1)	(88.9)	(25.7)
AFTER TAX
Other comprehensive income (loss) before reclassifications	622.7	259.0	(14.1)
Unrealized Gains (Losses) on Debt Securities Available for Sale
BEFORE TAX
Gains (Losses)	706.8	306.1	(31.9)
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.4	1.1	0.5
Net Change	707.2	307.2	(31.4)
TAX EFFECT
Gains (Losses)	(179.3)	(78.0)	9.2
Reclassification Adjustment for Losses (Gains) Included in Net Income	(0.1)	(0.3)	(0.1)
Net Change	(179.4)	(78.3)	9.1
AFTER TAX
Gains (Losses)	527.5	228.1	(22.7)
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.3	0.8	0.4
Other comprehensive income (loss) before reclassifications	527.8	228.9	(22.3)
Unrealized (Losses) Gains on Cash Flow Hedges
BEFORE TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(28.1)	(26.7)	(71.1)
Net Change	0.8	(10.3)	(1.8)
TAX EFFECT
Reclassification Adjustment for Losses (Gains) Included in Net Income	7.0	6.6	17.7
Net Change	(0.3)	2.6	0.4
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(21.1)	(20.1)	(53.4)
Other comprehensive income (loss) before reclassifications	0.5	(7.7)	(1.4)
Foreign Currency Adjustments
BEFORE TAX
Net Change	(7.5)	65.6	63.4
TAX EFFECT
Net Change	34.4	(15.7)	(41.2)
AFTER TAX
Other comprehensive income (loss) before reclassifications	26.9	49.9	22.2
Foreign Currency Translation Adjustments
BEFORE TAX
Net Change	169.1	6.4	(107.8)
TAX EFFECT
Net Change	(8.3)	(1.6)	1.5
AFTER TAX
Other comprehensive income (loss) before reclassifications	160.8	4.8	(106.3)
Long-Term Intra-Entity Foreign Currency Transaction (Losses) Gains
BEFORE TAX
Net Change	2.1	(0.5)	(1.8)
TAX EFFECT
Net Change	(0.5)	0.1	0.5
AFTER TAX
Other comprehensive income (loss) before reclassifications	1.6	(0.4)	(1.3)
Net Investment Hedge Gains (Losses)
BEFORE TAX
Net Change	(178.7)	59.7	173.0
TAX EFFECT
Net Change	43.2	(14.2)	(43.2)
AFTER TAX
Other comprehensive income (loss) before reclassifications	(135.5)	45.5	129.8
Pension and Other Postretirement Benefit Adjustments
BEFORE TAX
Gains (Losses)	47.4	(36.8)	(54.9)
Net Change	90.3	(14.6)	(18.6)
TAX EFFECT
Gains (Losses)	(12.3)	7.9	9.6
Net Change	(22.8)	2.5	6.0
AFTER TAX
Gains (Losses)	35.1	(28.9)	(45.3)
Other comprehensive income (loss) before reclassifications	67.5	(12.1)	(12.6)
Amortization of Net Actuarial Loss
BEFORE TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	43.0	22.4	36.6
TAX EFFECT
Reclassification Adjustment for Losses (Gains) Included in Net Income	(10.5)	(5.4)	(3.6)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	32.5	17.0	33.0
Amortization of Prior Service Cost
BEFORE TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(0.1)	(0.2)	(0.3)
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	(0.1)	(0.2)	(0.3)
Foreign Exchange Contracts \| Unrealized (Losses) Gains on Cash Flow Hedges
BEFORE TAX
Gains (Losses)	28.9	14.9	70.5
TAX EFFECT
Gains (Losses)	(7.3)	(3.7)	(17.6)
AFTER TAX
Gains (Losses)	21.6	11.2	52.9
Interest Rate Contracts \| Unrealized (Losses) Gains on Cash Flow Hedges
BEFORE TAX
Gains (Losses)	0.0	1.5	(1.2)
TAX EFFECT
Gains (Losses)	0.0	(0.3)	0.3
AFTER TAX
Gains (Losses)	$ 0.0	$ 1.2	$ (0.9)

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