Accumulated Other Comprehensive Income (Loss) - Other Comprehensive Income (Loss) Reclassification Adjustments (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	$ 258.9			$ (248.3)			$ 161.3	$ (200.5)
Reclassifications out of AOCI	327.2			5.5			332.7	13.9
Tax effect	(146.9)			57.2			(128.7)	45.9
Total other comprehensive income (loss), net of tax	439.2	$ (9.2)	$ (64.7)	(185.6)	$ (68.1)	$ 113.0	365.3	(140.7)
Unrealized Gains (Losses) on Debt Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	244.0			(233.6)			165.3	(199.9)
Reclassifications out of AOCI	326.6			5.1			331.0	12.4
Tax effect	(146.4)			56.9			(128.5)	46.3
Total other comprehensive income (loss), net of tax	424.2			(171.6)			367.8	(141.2)
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	14.9			(14.7)			(4.0)	(0.6)
Reclassifications out of AOCI	0.0			0.0			0.0	0.0
Tax effect	(0.4)			0.3			0.2	0.0
Total other comprehensive income (loss), net of tax	14.5			(14.4)			(3.8)	(0.6)
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	0.6			0.4			1.7	1.5
Tax effect	(0.1)			0.0			(0.4)	(0.4)
Total other comprehensive income (loss), net of tax	$ 0.5			$ 0.4			$ 1.3	$ 1.1