Income Taxes - Net Deferred Tax Liability (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Components of Deferred Tax Assets and Liabilities [Abstract]
Deferred revenue	$ 8.6	$ 8.3
Employee benefits	101.1	99.8
Bad debt reserves	8.7	11.8
Pension	13.3	12.6
Net operating loss carryforward	21.8	28.6
Foreign tax credit	3.8	4.7
Operating lease liabilities	52.3	58.6
Investments in affiliates	13.9	0.0
Securities	189.7	264.1
Other	12.2	11.2
Deferred tax assets before valuation allowance	425.4	499.7
Valuation allowance	(13.7)	(7.4)
Deferred tax assets	411.7	492.3
Depreciable and amortizable assets	275.8	282.2
Claims and related salvage	88.5	81.6
Investments in affiliates	0.0	25.8
Operating lease assets	47.7	52.8
Unremitted foreign earnings	13.2	11.7
Deferred tax liabilities	425.2	454.1
Net deferred tax asset		$ 38.2
Net deferred tax liability	$ (13.5)