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Commitments and Contingencies - Commitments (Details)
CAD in Millions, AUD in Millions, $ in Millions
 12 Months Ended
Jul. 30, 2015
USD ($)
Dec. 31, 2017
CAD
Dec. 31, 2016
CAD
Dec. 31, 2015
CAD
Dec. 31, 2017
USD ($)
Dec. 31, 2017
AUD
Nov. 17, 2017
AUD
Disclosure of Commitments [Line Items]              
Coal supply and mining agreements   CAD 608.0          
Long-term service agreements   280.0          
Non-cancellable operating leases   156.0          
Growth   27.0          
TransAlta Energy Transition Bill $ 55 36.0          
Total   1,229.0          
Minimum operating lease payments recognised as expense   7.0 CAD 9.0 CAD 9.0      
Sublease payments received (amounts less than)   1.0 CAD 1.0        
Commitment period for Energy Bill initiatives 9 years            
Funded portion of Energy Bill commitment | $         $ 28    
Amount withheld under dispute | AUD           AUD 58.2  
Amount withheld under dispute, tax | AUD           AUD 43.0  
Pending litigation claim | AUD             AUD 54.1
Natural gas, transportation, and other purchase contracts              
Disclosure of Commitments [Line Items]              
Purchase obligations   98.0          
Transmission              
Disclosure of Commitments [Line Items]              
Purchase obligations   24.0          
2018              
Disclosure of Commitments [Line Items]              
Coal supply and mining agreements   155.0          
Long-term service agreements   108.0          
Non-cancellable operating leases   9.0          
Growth   27.0          
TransAlta Energy Transition Bill   6.0          
Total   362.0          
2018 | Natural gas, transportation, and other purchase contracts              
Disclosure of Commitments [Line Items]              
Purchase obligations   48.0          
2018 | Transmission              
Disclosure of Commitments [Line Items]              
Purchase obligations   9.0          
2019              
Disclosure of Commitments [Line Items]              
Coal supply and mining agreements   159.0          
Long-term service agreements   50.0          
Non-cancellable operating leases   9.0          
Growth   0.0          
TransAlta Energy Transition Bill   6.0          
Total   237.0          
2019 | Natural gas, transportation, and other purchase contracts              
Disclosure of Commitments [Line Items]              
Purchase obligations   7.0          
2019 | Transmission              
Disclosure of Commitments [Line Items]              
Purchase obligations   6.0          
2020              
Disclosure of Commitments [Line Items]              
Coal supply and mining agreements   161.0          
Long-term service agreements   41.0          
Non-cancellable operating leases   9.0          
Growth   0.0          
TransAlta Energy Transition Bill   6.0          
Total   228.0          
2020 | Natural gas, transportation, and other purchase contracts              
Disclosure of Commitments [Line Items]              
Purchase obligations   5.0          
2020 | Transmission              
Disclosure of Commitments [Line Items]              
Purchase obligations   6.0          
2021              
Disclosure of Commitments [Line Items]              
Coal supply and mining agreements   23.0          
Long-term service agreements   31.0          
Non-cancellable operating leases   9.0          
Growth   0.0          
TransAlta Energy Transition Bill   6.0          
Total   77.0          
2021 | Natural gas, transportation, and other purchase contracts              
Disclosure of Commitments [Line Items]              
Purchase obligations   5.0          
2021 | Transmission              
Disclosure of Commitments [Line Items]              
Purchase obligations   3.0          
2022              
Disclosure of Commitments [Line Items]              
Coal supply and mining agreements   14.0          
Long-term service agreements   15.0          
Non-cancellable operating leases   9.0          
Growth   0.0          
TransAlta Energy Transition Bill   6.0          
Total   48.0          
2022 | Natural gas, transportation, and other purchase contracts              
Disclosure of Commitments [Line Items]              
Purchase obligations   4.0          
2022 | Transmission              
Disclosure of Commitments [Line Items]              
Purchase obligations   0.0          
2023 and thereafter              
Disclosure of Commitments [Line Items]              
Coal supply and mining agreements   96.0          
Long-term service agreements   35.0          
Non-cancellable operating leases   111.0          
Growth   0.0          
TransAlta Energy Transition Bill   6.0          
Total   277.0          
2023 and thereafter | Natural gas, transportation, and other purchase contracts              
Disclosure of Commitments [Line Items]              
Purchase obligations   29.0          
2023 and thereafter | Transmission              
Disclosure of Commitments [Line Items]              
Purchase obligations   0.0          
Line loss rule proceeding              
Disclosure of Commitments [Line Items]              
Estimated financial effect of contingent liabilities   15.0          
Provision for line loss rule proceeding   CAD 7.5