Components of Energy Supply Obligations (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	$ 16.1	$ 10.4	$ 12.1
Exchange Gas Obligation
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	9.7	4.4	6.0
Renewable Energy Portfolio Standards
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	6.4	5.7	5.8
Power Supply Contract Divestitures
Contractual Obligation [Line Items]
Energy Supply Obligations-Current	$ 0.0	$ 0.3	$ 0.3

Current liabilities attributable to energy marketing activities (trading activities). Current energy marketing (trading activities) are when entities engage in both financial and physical trading to increase profits, manage their commodity price risk and enhance system reliability. They may trade electricity, coal, natural gas and oil. These entities typically use a variety of financial instruments, including forward contracts, options and swaps.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.20)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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