Accumulated Other Comprehensive Income (Loss) - Details of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
PRE-TAX
Net Change	$ 228.5	$ 480.4	$ 551.5
TAX
Net Change	(5.0)	(156.5)	(120.2)
AFTER TAX
Net Change	223.5	323.9	431.3
Unrealized Gains (Losses) on Available for Sale Debt Securities
PRE-TAX
Gains (Losses)	162.3	147.7	319.4
Net Change	262.2	438.3	594.5
TAX
Gains (Losses)	(40.9)	(39.5)	(81.1)
Net Change	(65.4)	(112.5)	(150.8)
AFTER TAX
Gains (Losses)	121.4	108.2	238.3
Net Change	196.8	325.8	443.7
Interest income on debt securities
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	99.9	101.3	105.6
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(24.5)	(25.1)	(26.7)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	75.4	76.2	78.9
Net gains on debt securities
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	189.3	169.5
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	(47.9)	(43.0)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	0.0	141.4	126.5
Unrealized Gains (Losses) on Cash Flow Hedges
PRE-TAX
Gains (Losses)	20.9	13.2	36.3
Reclassification Adjustment for Losses (Gains) Included in Net Income	(20.5)	(13.4)	(36.8)
Net Change	0.4	(0.2)	(0.5)
TAX
Gains (Losses)	(5.1)	(3.2)	(9.2)
Reclassification Adjustment for Losses (Gains) Included in Net Income	5.0	3.2	9.3
Net Change	(0.1)	0.0	0.1
AFTER TAX
Gains (Losses)	15.8	10.0	27.1
Reclassification Adjustment for Losses (Gains) Included in Net Income	(15.5)	(10.2)	(27.5)
Net Change	0.3	(0.2)	(0.4)
Foreign Currency Adjustments
PRE-TAX
Net Change	(51.7)	83.2	22.5
TAX
Net Change	67.5	(53.7)	16.5
AFTER TAX
Net Change	15.8	29.5	39.0
Foreign Currency Translation Adjustments
PRE-TAX
Net Change	275.2	(149.0)	100.8
TAX
Net Change	(12.8)	5.4	(1.6)
AFTER TAX
Net Change	262.4	(143.6)	99.2
Long-Term Intra-Entity Foreign Currency Transaction Gains (Losses)
PRE-TAX
Net Change	1.2	(0.9)	(0.9)
TAX
Net Change	(0.2)	0.2	0.2
AFTER TAX
Net Change	1.0	(0.7)	(0.7)
Net Investment Hedge (Losses) Gains
PRE-TAX
Net Change	(328.1)	233.1	(77.4)
TAX
Net Change	80.5	(59.3)	17.9
AFTER TAX
Net Change	(247.6)	173.8	(59.5)
Pension and Other Postretirement Benefit Adjustments
PRE-TAX
Gains (Losses)	5.6	(54.0)	(71.4)
Net Change	17.6	(40.9)	(65.0)
TAX
Gains (Losses)	(4.0)	12.9	15.7
Net Change	(7.0)	9.7	14.0
AFTER TAX
Gains (Losses)	1.6	(41.1)	(55.7)
Net Change	10.6	(31.2)	(51.0)
Amortization of Net Actuarial Loss
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	12.2	13.0	6.4
TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(3.0)	(3.2)	(1.7)
AFTER TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	9.2	9.8	4.7
Amortization of Prior Service Cost (Credit)
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(0.1)	(0.1)	0.0
TAX
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)	(0.1)	0.0
Settlement Loss
PRE-TAX
Reclassification Adjustment for Losses (Gains) Included in Net Income	(0.1)	0.2	0.0
TAX
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	$ 0.0

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