Accumulated Other Comprehensive Income (Loss) - Other Comprehensive Income (Loss) Reclassification Adjustments (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2023	Mar. 31, 2022
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications		$ 144.6	$ (494.8)
Reclassifications out of AOCI		5.0	2.3
Tax effect		(36.6)	124.3
Total other comprehensive income (loss), net of tax		113.0	(368.2)
Unrealized Gains (Losses) on Debt Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications		142.0	(497.1)
Reclassifications out of AOCI		4.5	0.7
Tax effect		(36.4)	124.8
Total other comprehensive income (loss), net of tax		110.1	(371.6)
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications		2.6	2.3
Reclassifications out of AOCI		0.0	0.0
Tax effect		0.0	0.0
Total other comprehensive income (loss), net of tax		2.6	2.3
Pension Benefit Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Pretax change before reclassifications		0.0	0.0
Reclassifications out of AOCI	[1]	0.5	1.6
Tax effect		(0.2)	(0.5)
Total other comprehensive income (loss), net of tax		$ 0.3	$ 1.1

[1] Amounts are components of net periodic cost. See Note 11 Employee Benefit Plans for additional details.