Accumulated Other Comprehensive Income (Loss) - Other Comprehensive Income (Loss) Reclassification Adjustments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	$ 126.8	$ (85.2)
Reclassifications out of AOCI	2.0	4.0
Tax effect	(32.2)	16.5
Total other comprehensive income (loss), net of tax	96.6	(64.7)
Unrealized Gains (Losses) on Debt Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	122.9	(70.3)
Reclassifications out of AOCI	1.5	3.4
Tax effect	(32.1)	16.3
Total other comprehensive income (loss), net of tax	92.3	(50.6)
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	3.9	(14.9)
Reclassifications out of AOCI	0.0	0.0
Tax effect	0.0	0.4
Total other comprehensive income (loss), net of tax	3.9	(14.5)
Pension Benefit Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	0.0	0.0
Reclassifications out of AOCI	0.5	0.6
Tax effect	(0.1)	(0.2)
Total other comprehensive income (loss), net of tax	$ 0.4	$ 0.4