Derivative Instruments and Hedging Activities - Narrative (Details) kWh in Millions, MMBTU in Millions	3 Months Ended
Dec. 31, 2016 USD ($) kWh MMBTU	Sep. 30, 2016 USD ($) kWh MMBTU	Dec. 31, 2015 USD ($) kWh MMBTU
Derivative
Restricted cash in brokerage accounts	$ 0	$ 583,000	$ 6,324,000
IRPAs
Derivative
Notional amount of unsettled IRPA contracts	0		$ 290,000,000,000
Amount of net losses associated with interest rate hedges to be reclassified with interest rate hedges during the next 12 months	$ 3,451,000
Gas Utility
Derivative
Notional amount (energy measure) \| MMBTU	11.7	18.4	12.4
Maximum length of time hedged in price risk cash flow hedges (in months)	9 months
Electric Utility \| Electric Utility Electric Transmission Congestion
Derivative
Notional amount (energy measure) \| kWh	36.2	58.3	172.6
Maximum length of time hedged in price risk cash flow hedges (in months)	5 months

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

Maximum length of time hedged in price risk cash flow hedges.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

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- Definition

The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. For a classified balance sheet represents the current portion only (the noncurrent portion has a separate concept); there is a separate and distinct element for unclassified presentations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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