Leases - Maturities of Lease Liabilities (Details) $ in Millions	Mar. 31, 2019 CAD ($)
Leases [Abstract]
Finance Lease, Payments for remainder of fiscal year	$ 6
Finance Lease, Payments due year two	9
Finance Lease, Payments due year three	9
Finance Lease, Payments due year four	111
Finance Lease, Payments due year five	9
Finance Lease, Payments due after year five	30
Finance Lease, Total lease payments	174
Finance Lease, Imputed interest	19
Finance Lease, Present value of lease payments	155
Operating Lease, Payments due remainder of fiscal year	67
Operating Lease, Payments due year two	70
Operating Lease, Payments due year three	55
Operating Lease, Payments due year four	48
Operating Lease, Payments due year five	49
Operating Lease, Payments due after year five	154
Operating Lease, Total lease payments	443
Operating Lease, Imputed interest	57
Operating Lease, Present value of lease payments	$ 386