Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred tax liabilities
Property related	$ 1,549.1	$ 1,431.9
Pension and postretirement benefits	105.0	92.0
Pension	69.2	71.1
Total deferred tax liabilities	1,723.3	1,595.0
Deferred tax assets
Loss and credit carryforwards	91.3	80.0
Medical benefits	46.9	47.7
Insurance reserves	27.3	27.6
Pension and postretirement benefits	105.0	92.0
Capitalized energy conservation assistance costs	22.9	21.4
Other	23.3	17.5
Total deferred tax assets	316.7	286.2
Total deferred tax liability, net	$ 1,406.6	$ 1,308.8