Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets
Accrued estimated costs	$ 48.4	$ 43.2
Basis difference in assets and liabilities	68.5	59.0
Operating lease liabilities	12.3	12.9
Tax losses and credit carryforwards	1.7	3.0
Net cumulative defined benefit pension plan losses	81.5	95.9
Retiree medical and life insurance benefits	6.0	6.2
Total deferred tax assets	218.4	220.2
Deferred Tax Liabilities
Revenue recognition differences	116.7	77.7
Basis differences in intangible assets	9.2	8.6
ROU assets	11.4	12.0
Total deferred tax liabilities	137.3	98.3
Total net deferred tax assets	$ 81.1	$ 121.9