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Short Term Debt (short-term revolving and non-revolving credit facilities, outstanding borrowings and available capacity) (Details) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Short term debt revolving and non revolving credit facility [Member] |  |
Short-term Debt [Line Items] |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 2,024 | $ 1,762
Credit Facility Advances | 1,241 | 960
Letters of Credit Outstanding, Amount | 3 | 3
Total advances under available facilities | 1,244 | 963
Available capacity under existing agreements | 780 | 799
TECO Energy Inc. and TECO Finance Inc. [Member] | Revolving Credit Facility [Member] |  |
Short-term Debt [Line Items] |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 376 | $ 403
Line of Credit Facility, Expiration Date | Mar. 22, 2022 | Dec. 17, 2018
TECO Energy Inc. and TECO Finance Inc. [Member] | Finance Term Credit Facility [Member] |  |
Short-term Debt [Line Items] |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 502 | $ 537
Line of Credit Facility, Expiration Date | Dec. 31, 2018 |
Tampa Electric Division [Member] | Revolving Credit Facility [Member] |  |
Short-term Debt [Line Items] |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 408 | 436
Line of Credit Facility, Expiration Date | Mar. 22, 2022 |
Tampa Electric Division [Member] | Accounts Receivable Revolving Credit Facility |  |
Short-term Debt [Line Items] |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 188 | 201
Line of Credit Facility, Expiration Date | Dec. 31, 2018 |
Tampa Electric Division [Member] | Finance Term Credit Facility [Member] |  |
Short-term Debt [Line Items] |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 377 | 0
Line of Credit Facility, Expiration Date | Nov. 01, 2018 |
New Mexico Gas Company [Member] | Revolving Credit Facility [Member] |  |
Short-term Debt [Line Items] |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 157 | $ 168
Line of Credit Facility, Expiration Date | Mar. 22, 2022 | Dec. 17, 2018
Grand Bahama Power Company Limited [Member] | Revolving Credit Facility [Member] |  |
Short-term Debt [Line Items] |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 16 | $ 17

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- Definition Advances against the line of credit facility which can be made by way of overdraft. + References No definition available. + Details
Name: | ema_CreditFacilityAdvances
Namespace Prefix: | ema_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of borrowed funds under the facility. + References No definition available. + Details
Name: | ema_UseOfAvailableFacilities
Namespace Prefix: | ema_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Date the credit facility terminates, in CCYY-MM-DD format. + 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 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + 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 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + 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 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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. + Details
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Data Type: | xbrli:stringItemType
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Period Type: | duration

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- Details
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- Details
Name: | dei_LegalEntityAxis=ema_TecoEnergyIncAndTecoFinanceIncMember
Namespace Prefix: |
Data Type: | na
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- Details
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- Details
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- Details
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Namespace Prefix: |
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- Details
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Period Type: |