Accumulated Other Comprehensive Income (Loss) - Details of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
PRE-TAX
Net Change	$ 551.5	$ (2,004.9)	$ (590.2)
TAX
Net Change	(120.2)	471.3	126.6
AFTER TAX
Net Change	431.3	(1,533.6)	(463.6)
U.S. \| Pension Plans, Defined Benefit
AFTER TAX
Defined benefit plan, net periodic benefit cost (credit), gain (loss) due to settlement	0.0	(44.1)	(27.9)
Unrealized Gains (Losses) on Available for Sale Debt Securities
PRE-TAX
Gains (Losses)	319.4	(2,263.8)	(685.0)
Net Change	594.5	(1,984.7)	(721.3)
TAX
Gains (Losses)	(81.1)	580.2	177.5
Net Change	(150.8)	510.0	186.6
AFTER TAX
Gains (Losses)	238.3	(1,683.6)	(507.5)
Net Change	443.7	(1,474.7)	(534.7)
Interest income on debt securities
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	105.6	65.1	(36.6)
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(26.7)	(16.4)	9.2
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	78.9	48.7	(27.4)
Net gains on debt securities
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	169.5	214.0	0.3
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(43.0)	(53.8)	(0.1)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	126.5	160.2	0.2
Unrealized (Losses) Gains on Cash Flow Hedges
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(36.8)	3.7	(4.5)
Net Change	(0.5)	4.7	1.1
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	9.3	(0.8)	1.1
Net Change	0.1	(1.1)	(0.3)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(27.5)	2.9	(3.4)
Net Change	(0.4)	3.6	0.8
Foreign Currency Adjustments
PRE-TAX
Net Change	22.5	76.0	53.4
TAX
Net Change	16.5	(66.6)	(42.9)
AFTER TAX
Net Change	39.0	9.4	10.5
Foreign Currency Translation Adjustments
PRE-TAX
Net Change	100.8	(200.1)	(133.8)
TAX
Net Change	(1.6)	3.0	4.1
AFTER TAX
Net Change	99.2	(197.1)	(129.7)
Long-Term Intra-Entity Foreign Currency Transaction (Losses) Gains
PRE-TAX
Net Change	(0.9)	(2.6)	0.6
TAX
Net Change	0.2	0.7	(0.1)
AFTER TAX
Net Change	(0.7)	(1.9)	0.5
Net Investment Hedge Gains (Losses)
PRE-TAX
Net Change	(77.4)	278.7	186.6
TAX
Net Change	17.9	(70.3)	(46.9)
AFTER TAX
Net Change	(59.5)	208.4	139.7
Pension and Other Postretirement Benefit Adjustments
PRE-TAX
Gains (Losses)	(71.4)	(163.9)	7.9
Net Change	(65.0)	(100.9)	76.6
TAX
Gains (Losses)	15.7	44.7	0.2
Net Change	14.0	29.0	(16.8)
AFTER TAX
Gains (Losses)	(55.7)	(119.2)	8.1
Net Change	(51.0)	(71.9)	59.8
Amortization of Net Actuarial Loss
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	6.4	19.1	41.4
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(1.7)	(4.7)	(10.3)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	4.7	14.4	31.1
Amortization of Prior Service Cost (Credit)
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	(1.0)	(1.0)
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	0.3	0.3
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	(0.7)	(0.7)
Settlement Loss
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	44.9	28.3
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	(11.3)	(7.0)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	33.6	21.3
Foreign Exchange Contracts \| Unrealized (Losses) Gains on Cash Flow Hedges
PRE-TAX
Gains (Losses)	36.3	1.0	5.6
TAX
Gains (Losses)	(9.2)	(0.3)	(1.4)
AFTER TAX
Gains (Losses)	$ 27.1	$ 0.7	$ 4.2

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