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Commitments and Contingencies - Commitments (Details)
$ in Millions, $ in Millions
 12 Months Ended
Jul. 30, 2015
USD ($)
Dec. 31, 2018
CAD ($)
legal_dispute
Dec. 31, 2017
CAD ($)
Dec. 31, 2016
CAD ($)
Dec. 31, 2018
USD ($)
Dec. 17, 2018
Disclosure of Commitments [Line Items]            
Coal supply and mining agreements   $ 488.0        
Long-term service agreements   241.0        
Non-cancellable operating leases   80.0        
Growth   547.0        
TransAlta Energy Transition Bill $ 55 31.0        
Total   1,655.0        
Minimum operating lease payments recognised as expense   8.0 $ 7.0 $ 9.0    
Sublease payments received (amounts less than)   $ 1.0 $ 1.0 $ 1.0    
Commitment period for Energy Bill initiatives 9 years          
Funded portion of Energy Bill commitment         $ 33  
Number Of legal disputes | legal_dispute   2        
Litigation settlement, amount awarded from other party   $ 157.0        
Mining assets   56.0        
Natural gas, transportation and other purchase contracts            
Disclosure of Commitments [Line Items]            
Purchase obligations   236.0        
Transmission            
Disclosure of Commitments [Line Items]            
Purchase obligations   32.0        
2018            
Disclosure of Commitments [Line Items]            
Coal supply and mining agreements   158.0        
Long-term service agreements   64.0        
Non-cancellable operating leases   8.0        
Growth   324.0        
TransAlta Energy Transition Bill   6.0        
Total   597.0        
2018 | Natural gas, transportation and other purchase contracts            
Disclosure of Commitments [Line Items]            
Purchase obligations   28.0        
2018 | Transmission            
Disclosure of Commitments [Line Items]            
Purchase obligations   9.0        
2019            
Disclosure of Commitments [Line Items]            
Coal supply and mining agreements   160.0        
Long-term service agreements   86.0        
Non-cancellable operating leases   8.0        
Growth   79.0        
TransAlta Energy Transition Bill   7.0        
Total   365.0        
2019 | Natural gas, transportation and other purchase contracts            
Disclosure of Commitments [Line Items]            
Purchase obligations   15.0        
2019 | Transmission            
Disclosure of Commitments [Line Items]            
Purchase obligations   10.0        
2020            
Disclosure of Commitments [Line Items]            
Coal supply and mining agreements   27.0        
Long-term service agreements   32.0        
Non-cancellable operating leases   8.0        
Growth   144.0        
TransAlta Energy Transition Bill   6.0        
Total   236.0        
2020 | Natural gas, transportation and other purchase contracts            
Disclosure of Commitments [Line Items]            
Purchase obligations   13.0        
2020 | Transmission            
Disclosure of Commitments [Line Items]            
Purchase obligations   6.0        
2021            
Disclosure of Commitments [Line Items]            
Coal supply and mining agreements   24.0        
Long-term service agreements   17.0        
Non-cancellable operating leases   7.0        
Growth   0.0        
TransAlta Energy Transition Bill   6.0        
Total   69.0        
2021 | Natural gas, transportation and other purchase contracts            
Disclosure of Commitments [Line Items]            
Purchase obligations   11.0        
2021 | Transmission            
Disclosure of Commitments [Line Items]            
Purchase obligations   4.0        
2022            
Disclosure of Commitments [Line Items]            
Coal supply and mining agreements   24.0        
Long-term service agreements   8.0        
Non-cancellable operating leases   4.0        
Growth   0.0        
TransAlta Energy Transition Bill   6.0        
Total   57.0        
2022 | Natural gas, transportation and other purchase contracts            
Disclosure of Commitments [Line Items]            
Purchase obligations   12.0        
2022 | Transmission            
Disclosure of Commitments [Line Items]            
Purchase obligations   3.0        
2023 and thereafter            
Disclosure of Commitments [Line Items]            
Coal supply and mining agreements   95.0        
Long-term service agreements   34.0        
Non-cancellable operating leases   45.0        
Growth   0.0        
TransAlta Energy Transition Bill   0.0        
Total   331.0        
2023 and thereafter | Natural gas, transportation and other purchase contracts            
Disclosure of Commitments [Line Items]            
Purchase obligations   157.0        
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   $ 7.5        
Tidewater Brazeau River Pipeline | Tidewater            
Disclosure of Commitments [Line Items]            
Future option to acquire pipeline, ownership percentage           50.00%