Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets
Accrued estimated costs	$ 56.1	$ 82.0
Basis difference in assets and liabilities	2.1	8.5
Tax losses and credit carryforwards	12.9	6.5
Net cumulative defined benefit pension plan losses	114.7	212.9
Retiree medical and life insurance benefits	9.2	16.2
Valuation allowance	(1.7)	(1.7)
Total deferred tax assets	193.3	324.4
Deferred Tax Liabilities
Revenue recognition differences	40.4	21.7
Basis differences in intangible assets	7.1	10.2
Total deferred tax liabilities	47.5	31.9
Total net deferred tax assets	$ 145.8	$ 292.5