Leases - Maturities of Lease Liabilities (Details) $ in Millions	Jun. 30, 2019 CAD ($)
Leases [Abstract]
Finance lease, Payments due remainder of year one	$ 5
Finance lease, Payments due year two	11
Finance lease, Payments due year three	9
Finance lease, Payments due year four	109
Finance lease, Payments due year five	9
Finance lease, Payments due after year five	30
Finance lease, Total lease payments	173
Finance lease, Imputed interest	18
Finance lease, Present value of lease payments	155
Operating lease, Payments due remainder of year one	50
Operating lease, Payments due year two	72
Operating lease, Payments due year three	55
Operating lease, Payments due year four	47
Operating lease, Payments due year five	48
Operating lease, Payments due after year five	149
Operating lease, Total lease payments	421
Operating lease, Imputed interest	46
Operating lease, Present value of lease payments	$ 375