Leases - Maturities of Lease Liabilities (Details) $ in Millions	Sep. 30, 2019 CAD ($)
Leases [Abstract]
Finance lease, Payments due remainder of year one	$ 3
Finance lease, Payments due year two	11
Finance lease, Payments due year three	10
Finance lease, Payments due year four	110
Finance lease, Payments due year five	9
Finance lease, Payments due after year five	29
Finance lease, Total lease payments	172
Finance lease, Imputed interest	17
Finance lease, Present value of lease payments	155
Operating lease, Payments due remainder of year one	28
Operating lease, Payments due year two	69
Operating lease, Payments due year three	54
Operating lease, Payments due year four	52
Operating lease, Payments due year five	40
Operating lease, Payments due after year five	178
Operating lease, Total lease payments	421
Operating lease, Imputed interest	51
Operating lease, Present value of lease payments	$ 370