Income Taxes (Tax Pool Amounts) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Canada [Member]
Tax Credit Carryforward [Line Items]
Tax Pools / Tax Basis	$ 1,520
Operating Loss Carryforwards	982
Canada [Member] | Capital Loss Carryforward [Member]
Tax Credit Carryforward [Line Items]
Other Tax Carryforward, Gross Amount	$ 15
Canada [Member] | Charitable Donations [Member]
Tax Credit Carryforward [Line Items]
Other Tax Carryforward, Expiration Dates	Dec. 31, 2022
Other Tax Carryforward, Gross Amount	$ 1
United States [Member]
Tax Credit Carryforward [Line Items]
Tax Pools / Tax Basis	4,703
Operating Loss Carryforwards	3,407
United States [Member] | Charitable Donations [Member]
Tax Credit Carryforward [Line Items]
Other Tax Carryforward, Gross Amount	13
United States [Member] | Foreign Tax Credits [Member]
Tax Credit Carryforward [Line Items]
Other Tax Carryforward, Gross Amount	$ 198
Maximum [Member] | Canada [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Dates	Dec. 31, 2037
Maximum [Member] | United States [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Dates	Dec. 31, 2037
Maximum [Member] | United States [Member] | Charitable Donations [Member]
Tax Credit Carryforward [Line Items]
Other Tax Carryforward, Expiration Dates	Dec. 31, 2023
Maximum [Member] | United States [Member] | Foreign Tax Credits [Member]
Tax Credit Carryforward [Line Items]
Other Tax Carryforward, Expiration Dates	Dec. 31, 2025
Minimum [Member] | Canada [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Dates	Dec. 31, 2027
Minimum [Member] | United States [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Dates	Dec. 31, 2031
Minimum [Member] | United States [Member] | Charitable Donations [Member]
Tax Credit Carryforward [Line Items]
Other Tax Carryforward, Expiration Dates	Dec. 31, 2019
Minimum [Member] | United States [Member] | Foreign Tax Credits [Member]
Tax Credit Carryforward [Line Items]
Other Tax Carryforward, Expiration Dates	Dec. 31, 2021