Variable Interest Entities (Narrative) (Details) MMcf in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) MMcf	Dec. 31, 2016 USD ($)
Variable Interest Entity [Line Items]
Accounts Payable and Accrued Liabilities, Current	$ 1,415	$ 1,303
Production Field Centre [Member]
Variable Interest Entity [Line Items]
Number of Terms	12
Lease Expiration Date	Dec. 01, 2021
Capital Lease Obligations	$ 314	$ 299
Length of Renewal Term, in Years	1 year
Veresen Midstream Limited Partnership [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Financial Support, Term of Assessment	8 years
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	$ 2,344
Veresen Midstream Limited Partnership [Member] | Minimum [Member]
Variable Interest Entity [Line Items]
Length of Remaining Terms, In Years	15 years
Veresen Midstream Limited Partnership [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Length of Remaining Terms, In Years	28 years
Length of Renewal Term, in Years	10 years
Veresen Midstream Limited Partnership [Member] | Natural gas gathering and compression [Member]
Variable Interest Entity [Line Items]
Contracted Capacity Volumes | MMcf	630
Veresen Midstream Limited Partnership [Member] | Natural gas processing [Member]
Variable Interest Entity [Line Items]
Contracted Capacity Volumes | MMcf	772
Take or Pay Commitment [Member] | Veresen Midstream Limited Partnership [Member]
Variable Interest Entity [Line Items]
Accounts Payable and Accrued Liabilities, Current	$ 0