Accumulated Other Comprehensive Income (Loss) (Other Comprehensive Income (Loss) Reclassification Adjustments) (Detail) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications		$ (96.8)		$ (387.4)		$ 47.8	$ (882.2)
Reclassifications out of AOCI		3.4		8.4		8.4	10.7
Tax effect		25.3		88.8		(11.3)	213.1
Total other comprehensive (loss) income, net of tax		(68.1)	$ 113.0	(290.2)	$ (368.2)	44.9	(658.4)
Unrealized Gains (Losses) on Debt Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications		(108.3)		(362.3)		33.7	(859.4)
Reclassifications out of AOCI		2.8		6.9		7.3	7.6
Tax effect		25.8		88.6		(10.6)	213.4
Total other comprehensive (loss) income, net of tax		(79.7)		(266.8)		30.4	(638.4)
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications		11.5		(25.1)		14.1	(22.8)
Reclassifications out of AOCI		0.0		0.0		0.0	0.0
Tax effect		(0.3)		0.6		(0.3)	0.6
Total other comprehensive (loss) income, net of tax		11.2		(24.5)		13.8	(22.2)
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	[1]	0.6		1.5		1.1	3.1
Tax effect		(0.2)		(0.4)		(0.4)	(0.9)
Total other comprehensive (loss) income, net of tax		$ 0.4		$ 1.1		$ 0.7	$ 2.2

[1] Amounts are components of net periodic cost. See Note 12 Employee Benefit Plans for additional details.