Accumulated Other Comprehensive Income (Loss) (Reclassifications Out of AOCI) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss				$ (1.1)
Pretax total		$ (3.4)	$ (8.4)	(8.4)	$ (10.7)
Unrealized Gains (Losses) on Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized losses on sales of debt securities		(2.8)	(6.9)	(7.3)	(7.6)
Tax effect		0.8	1.7	1.9	1.9
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss	[1]	(0.6)	(1.4)	(1.1)	(2.9)
Amortization of prior service cost	[1]	0.0	(0.1)	0.0	(0.2)
Pretax total	[1]	(0.6)	(1.5)	(1.1)	(3.1)
Tax effect	[1]	$ 0.2	$ 0.4	$ 0.4	$ 0.9

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