Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deferred Tax Assets
Accrued estimated costs	$ 49.5	$ 48.4
Basis difference in assets and liabilities	58.8	68.5
Operating lease liabilities	13.7	12.3
Tax losses and credit carryforwards	0.4	1.7
Net cumulative defined benefit pension plan losses	58.8	81.5
Retiree medical and life insurance benefits	4.7	6.0
Total deferred tax assets	185.9	218.4
Deferred Tax Liabilities
Revenue recognition differences	104.8	116.7
Basis differences in intangible assets	10.7	9.2
ROU assets	12.9	11.4
Total deferred tax liabilities	128.4	137.3
Total net deferred tax assets	$ 57.5	$ 81.1