Accumulated Other Comprehensive Income (Loss) - Details of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
PRE-TAX
Net Change	$ 480.4	$ 551.5	$ (2,004.9)
TAX
Net Change	(156.5)	(120.2)	471.3
AFTER TAX
Net Change	323.9	431.3	(1,533.6)
U.S. \| Pension Plans
AFTER TAX
Defined benefit plan, net periodic benefit cost (credit), gain (loss) due to settlement	0.0	0.0	(44.1)
Unrealized Gains (Losses) on Available for Sale Debt Securities
PRE-TAX
Gains (Losses)	147.7	319.4	(2,263.8)
Net Change	438.3	594.5	(1,984.7)
TAX
Gains (Losses)	(39.5)	(81.1)	580.2
Net Change	(112.5)	(150.8)	510.0
AFTER TAX
Gains (Losses)	108.2	238.3	(1,683.6)
Net Change	325.8	443.7	(1,474.7)
Interest income on debt securities
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	101.3	105.6	65.1
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(25.1)	(26.7)	(16.4)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	76.2	78.9	48.7
Net gains on debt securities
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	189.3	169.5	214.0
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(47.9)	(43.0)	(53.8)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	141.4	126.5	160.2
Unrealized (Losses) Gains on Cash Flow Hedges
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(13.4)	(36.8)	3.7
Net Change	(0.2)	(0.5)	4.7
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	3.2	9.3	(0.8)
Net Change	0.0	0.1	(1.1)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(10.2)	(27.5)	2.9
Net Change	(0.2)	(0.4)	3.6
Foreign Currency Adjustments
PRE-TAX
Net Change	83.2	22.5	76.0
TAX
Net Change	(53.7)	16.5	(66.6)
AFTER TAX
Net Change	29.5	39.0	9.4
Foreign Currency Translation Adjustments
PRE-TAX
Net Change	(149.0)	100.8	(200.1)
TAX
Net Change	5.4	(1.6)	3.0
AFTER TAX
Net Change	(143.6)	99.2	(197.1)
Long-Term Intra-Entity Foreign Currency Transaction (Losses) Gains
PRE-TAX
Net Change	(0.9)	(0.9)	(2.6)
TAX
Net Change	0.2	0.2	0.7
AFTER TAX
Net Change	(0.7)	(0.7)	(1.9)
Net Investment Hedge Gains (Losses)
PRE-TAX
Net Change	233.1	(77.4)	278.7
TAX
Net Change	(59.3)	17.9	(70.3)
AFTER TAX
Net Change	173.8	(59.5)	208.4
Pension and Other Postretirement Benefit Adjustments
PRE-TAX
Gains (Losses)	(54.0)	(71.4)	(163.9)
Net Change	(40.9)	(65.0)	(100.9)
TAX
Gains (Losses)	12.9	15.7	44.7
Net Change	9.7	14.0	29.0
AFTER TAX
Gains (Losses)	(41.1)	(55.7)	(119.2)
Net Change	(31.2)	(51.0)	(71.9)
Amortization of Net Actuarial Loss
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	13.0	6.4	19.1
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(3.2)	(1.7)	(4.7)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	9.8	4.7	14.4
Amortization of Prior Service Cost (Credit)
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(0.1)	0.0	(1.0)
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	0.0	0.3
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(0.1)	0.0	(0.7)
Settlement Loss
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.2	0.0	44.9
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	0.0	(11.3)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.2	0.0	33.6
Foreign Exchange Contracts \| Unrealized (Losses) Gains on Cash Flow Hedges
PRE-TAX
Gains (Losses)	13.2	36.3	1.0
TAX
Gains (Losses)	(3.2)	(9.2)	(0.3)
AFTER TAX
Gains (Losses)	$ 10.0	$ 27.1	$ 0.7

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