Accumulated Other Comprehensive Income (Loss) - Other Comprehensive Income (Loss) Reclassification Adjustments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	$ (85.2)	$ 144.6
Reclassifications out of AOCI	4.0	5.0
Tax effect	16.5	(36.6)
Total other comprehensive income, net of tax	(64.7)	113.0
Unrealized Gains (Losses) on Debt Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	(70.3)	142.0
Reclassifications out of AOCI	3.4	4.5
Tax effect	16.3	(36.4)
Total other comprehensive income, net of tax	(50.6)	110.1
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	(14.9)	2.6
Reclassifications out of AOCI	0.0	0.0
Tax effect	0.4	0.0
Total other comprehensive income, net of tax	(14.5)	2.6
Pension Benefit Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications	0.0	0.0
Reclassifications out of AOCI	0.6	0.5
Tax effect	(0.2)	(0.2)
Total other comprehensive income, net of tax	$ 0.4	$ 0.3