The amount recognised as a reduction of the carrying amount of an asset or cash-generating unit to its recoverable amount. [Refer: Carrying amount [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 36
-IssueDate 2017-01-01
-Paragraph 130
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=36&date=2017-03-01&anchor=para_130_b&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 36
-IssueDate 2017-01-01
-Paragraph 130
-Subparagraph d
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=36&date=2017-03-01&anchor=para_130_d_ii&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_ImpairmentLoss
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent reporting 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

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

+ Details

Name:	invest_InvestmentLineItems
Namespace Prefix:	invest_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Capacity Of Generation Units, Power

+ References

No definition available.

+ Details

Name:	tac_CapacityOfPowerGenerationAssetsPower
Namespace Prefix:	tac_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	instant

- Definition

Natural Gas Pipeline Capacity, Expansion Capability, Volume

+ References

No definition available.

+ Details

Name:	tac_NaturalGasPipelineCapacityExpansionCapabilityVolume
Namespace Prefix:	tac_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

- Definition

Natural Gas Pipeline Capacity, Volume

+ References

No definition available.

+ Details

Name:	tac_NaturalGasPipelineCapacityVolume
Namespace Prefix:	tac_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

- Definition

Natural Gas Pipeline, Length

+ References

No definition available.

+ Details

Name:	tac_NaturalGasPipelineLength
Namespace Prefix:	tac_
Data Type:	num:lengthItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of Pipelines Needed to Reduce Operational Risk

+ References

No definition available.

+ Details

Name:	tac_NumberofPipelinesNeededtoReduceOperationalRisk
Namespace Prefix:	tac_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Option to Acquire Pipeline, Ownership Percentage

+ References

No definition available.

+ Details

Name:	tac_OptiontoAcquirePipelineOwnershipPercentage
Namespace Prefix:	tac_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Property, Plant, And Equipment Useful Life1

+ References

No definition available.

+ Details

Name:	tac_PropertyPlantAndEquipmentUsefulLife1
Namespace Prefix:	tac_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Transition to Gas and Renewables Generation, Mothballing Term

+ References

No definition available.

+ Details

Name:	tac_TransitiontoGasandRenewablesGenerationMothballingTerm
Namespace Prefix:	tac_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	ifrs-full_TypesOfInvestmentPropertyAxis=tac_SundancePowerPlantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInvestmentPropertyAxis=tac_SundanceUnits1and2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInvestmentPropertyAxis=tac_SundanceUnit1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInvestmentPropertyAxis=tac_SundanceUnit2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInvestmentPropertyAxis=tac_SundanceUnit3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInvestmentPropertyAxis=tac_SundanceUnit4Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInvestmentPropertyAxis=tac_SundanceUnit5Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis=tac_CoalfiredFleetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CounterpartiesAxis=tac_TidewaterMidstreamandInfrastructureLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInvestmentPropertyAxis=tac_TidewaterBrazeauRiverPipelineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis=tac_GasConversionUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_TopOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Significant Events - Transition to Clean Power in Alberta (Details) MMcf in Millions, CAD in Millions		3 Months Ended	12 Months Ended
	Dec. 06, 2017 km MMcf	Jun. 30, 2017 CAD	Dec. 31, 2017 pipeline MW
Investment [Line Items]
Number of pipelines needed to reduce operational risk (at least) \| pipeline			2
Sundance Power Plant
Investment [Line Items]
Capacity of facility (in megawatts) \| MW			2,141
Sundance Units 1 and 2
Investment [Line Items]
Capacity of facility (in megawatts) \| MW			560
Sundance Unit 1
Investment [Line Items]
Impairment loss \| CAD		CAD 20
Sundance Unit 2
Investment [Line Items]
Transition to Gas and Renewables Generation, Mothballing Term	2 years
Sundance Unit 3
Investment [Line Items]
Transition to Gas and Renewables Generation, Mothballing Term	2 years
Sundance Unit 4
Investment [Line Items]
Transition to Gas and Renewables Generation, Mothballing Term	2 years
Sundance Unit 5
Investment [Line Items]
Transition to Gas and Renewables Generation, Mothballing Term	1 year
Coal-fired fleet
Investment [Line Items]
PP&E useful life			75 years
Tidewater \| Tidewater Brazeau River Pipeline
Investment [Line Items]
Natural gas pipeline, length (km) \| km	120
Natural gas pipeline capacity (mmcf) \| MMcf	130
Natural gas pipeline capacity, expansion capability (mmcf) \| MMcf	340
Future option to acquire pipeline, ownership percentage	50.00%
Minimum \| Gas conversion units
Investment [Line Items]
PP&E useful life			5 years
Maximum \| Gas conversion units
Investment [Line Items]
PP&E useful life			10 years