Long-Term Debt (Narrative) (Details) CAD in Millions, $ in Millions		12 Months Ended			24 Months Ended
	Mar. 30, 2016 USD ($) $ / loan	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Mar. 16, 2016 USD ($)	Dec. 31, 2015 CAD	Dec. 31, 2015 USD ($)	Apr. 06, 2015
Debt Instrument [Line Items]
Standby Fees		$ 15	$ 14	$ 11
Shelf Prospectus Remaining Borrowing Capacity		4,800			$ 4,800
Debt Instrument, Repurchase Amount	$ 400
Other		42	58	(27)
Discount Capitalized					0
Current portion of long-term debt		0			0
Long-term Debt Carrying Value		4,197	4,198		4,197
Long-term Debt, Fair Value		$ 5,042	4,553		$ 5,042
Amortization of Debt Remaining Life In Years		13 years
Interest Expense on Debt		$ 267	$ 296	497
Unsecured Notes, Due August 15, 2037 [Member] | 6.625% Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	$ 38
Debt Instrument, Interest Rate, Stated Percentage	6.625%	6.625%			6.625%
Unsecured Notes, Due February 1, 2038 [Member] | 6.50% Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	$ 295
Debt Instrument, Interest Rate, Stated Percentage	6.50%	6.50%			6.50%
Unsecured Notes, Due November 15, 2041 [Member] | 5.15% Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	$ 156
Debt Instrument, Interest Rate, Stated Percentage	5.15%	5.15%			5.15%
Unsecured Notes, Due December 1, 2017 [Member] | 5.90% Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount								$ 700
Debt Instrument, Interest Rate, Stated Percentage							5.90%	5.90%	5.90%
Unsecured Notes, Due January 18, 2018 [Member] | 5.80% Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount | CAD							CAD 750
Debt Instrument, Interest Rate, Stated Percentage							5.80%	5.80%	5.80%
2015 Early Debt Redemption [Member]
Debt Instrument [Line Items]
Interest Expense on Debt				$ 165
2016 Early Partial Debt Retirement Initial Announcement [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount						$ 250
2016 Early Partial Debt Retirement [Member]
Debt Instrument [Line Items]
Payments of Debt Extinguishment Costs	$ 14
Debt Instrument, Gain (Loss) on Repurchase	103
Other	$ 89
Early Tender Premium Per One Thousand Principal Amount | $ / loan	30
Repayment of long-term debt, including interest payable	$ 406
2016 Early Partial Debt Retirement [Member] | Interest Payable [Member]
Debt Instrument [Line Items]
Repayment of long-term debt, including interest payable	$ 6
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 4,500			$ 4,500
Expiration Date - Line of Credit		Jul. 31, 2020
Equity Adjustment, Cumulative Historical Ceiling Test Impairment		$ 7,700			$ 7,700
Maximum Number of Extensions Per Year		1
Maximum Period of Years Line of Credit Can Extended		5 years
Additional Period In Days Added To Extension		90 days
Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Financing Debt-to-Adjusted Capitalization, Percentage		0.60			0.60
Revolving Credit Facility [Member] | Parent Company [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 3,000			$ 3,000
United States [Member] | Revolving Credit Facility [Member] | Subsidiary Issuer [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 1,500			$ 1,500