Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets
Accrued estimated costs	$ 89.1	$ 113.3
Basis difference in assets and liabilities	8.5	6.0
Tax losses and credit carryforwards	6.5	3.8
Net cumulative defined benefit pension plan losses	212.9	227.8
Retiree medical and life insurance benefits	16.2	19.6
Valuation allowance	(1.7)	(1.2)
Total deferred tax assets	331.5	369.3
Deferred Tax Liabilities
Revenue recognition differences	21.7	30.7
Basis differences in intangible assets	17.3	13.8
Total deferred tax liabilities	39.0	44.5
Total net deferred tax assets	$ 292.5	$ 324.8