Fair Value Amount of Derivative Assets Liabilities Offset in Regulatory Liabilities Assets (Detail) - Not Designated as Hedging Instruments - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative Assets
Derivative Assets		$ 0.4
Derivative Liabilities
Derivative Liabilities	$ 0.0	0.0
Natural Gas Futures Contract | Prepayments and Other
Derivative Assets
Derivative Assets		0.1
Natural Gas Futures Contract | Other Noncurrent Assets
Derivative Assets
Derivative Assets		0.3
Natural Gas Futures Contract | Other Current Liabilities
Derivative Liabilities
Derivative Liabilities	0.0	0.0
Natural Gas Futures Contract | Other Noncurrent Liabilities
Derivative Liabilities
Derivative Liabilities	$ 0.0	$ 0.0