Components of Energy Supply Obligations (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	$ 15.0	$ 9.7	$ 12.9
Power Supply Contract Divestitures, Noncurrent	0.7	0.9	1.0
Total Energy Supply Obligations	15.7	10.6	13.9
Exchange Gas Obligation
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	9.5	5.4	8.9
Renewable Energy Portfolio Standards
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	5.2	4.0	3.7
Power Supply Contract Divestitures
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	$ 0.3	$ 0.3	$ 0.3