Variable Interest Entities (Pipeline Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entities [Line Items]
Investments in joint ventures	$ 1,602	$ 1,409
Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Investments in joint ventures	$ 1,235	1,060
Atlantic Coast Pipeline [Member]
Variable Interest Entities [Line Items]
Ownership percentage	47.00%
Atlantic Coast Pipeline [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Investments in joint ventures	$ 973	797
Constitution Pipeline [Member]
Variable Interest Entities [Line Items]
Ownership percentage	24.00%
Constitution Pipeline [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Investments in joint ventures	$ 25	25
Pipeline Investments [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Investments in joint ventures	$ 998	$ 822