Components of Current and Noncurrent Energy Supply Obligations (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Contractual Obligation [Line Items]
Energy Supply Obligations - Current	$ 12.0	$ 14.6
Power Supply Contract Divestitures, Noncurrent	1.3	1.6
Total Energy Supply Obligations	13.3	16.2
Exchange Gas Obligation
Contractual Obligation [Line Items]
Energy Supply Obligations - Current	7.8	10.3
Renewable Energy Portfolio Standards
Contractual Obligation [Line Items]
Energy Supply Obligations - Current	3.9	4.0
Power Supply Contract Divestitures
Contractual Obligation [Line Items]
Energy Supply Obligations - Current	$ 0.3	$ 0.3