Accumulated Other Comprehensive Income (Loss) - Other Comprehensive Income (Loss) Reclassification Adjustments (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2025	Jun. 30, 2024
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	$ 55.6		$ (12.4)		$ 182.4	$ (97.6)
Reclassifications out of AOCI	5.8		1.5		7.8	5.5
Tax effect	(7.2)		1.7		(39.4)	18.2
Total other comprehensive income (loss), net of tax	54.2	$ 96.6	(9.2)	$ (64.7)	150.8	(73.9)
Unrealized Gains (Losses) on Debt Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	19.9		(8.4)		142.8	(78.7)
Reclassifications out of AOCI	5.4		1.0		6.9	4.4
Tax effect	(6.3)		1.6		(38.4)	17.9
Total other comprehensive income (loss), net of tax	19.0		(5.8)		111.3	(56.4)
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	35.7		(4.0)		39.6	(18.9)
Reclassifications out of AOCI	0.0		0.0		0.0	0.0
Tax effect	(0.8)		0.2		(0.8)	0.6
Total other comprehensive income (loss), net of tax	34.9		(3.8)		38.8	(18.3)
Pension Benefit Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	0.0		0.0		0.0	0.0
Reclassifications out of AOCI	0.4		0.5		0.9	1.1
Tax effect	(0.1)		(0.1)		(0.2)	(0.3)
Total other comprehensive income (loss), net of tax	$ 0.3		$ 0.4		$ 0.7	$ 0.8