Components of Energy Supply Obligations (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	$ 9.7	$ 12.0
Power Supply Contract Divestitures, Noncurrent	0.9	1.3
Total Energy Supply Obligations	10.6	13.3
Exchange Gas Obligation
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	5.4	7.8
Renewable Energy Portfolio Standards
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	4.0	3.9
Power Supply Contract Divestitures
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	$ 0.3	$ 0.3

Fair value of liability arising from energy marketing activities (trading activities), expected to be settled after one year or beyond the normal operating cycle, if longer. Examples of energy marketing activities include, but are not limited to, trading of electricity, coal, natural gas and oil using financial instruments, including, but not limited to, forward contracts, options and swaps.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Name:	us-gaap_EnergyMarketingContractLiabilitiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant