Exit Costs - Transportation Commitments - Income Statement Disclosures (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Long-term Purchase Commitment [Line Items]
Revenue from Sales	$ 1,052	$ 1,286
Corporate
Long-term Purchase Commitment [Line Items]
Revenue from Sales	27	31
Purchased Gas
Long-term Purchase Commitment [Line Items]
Revenue from Sales	74	53
Costs and Expenses	87	57
Purchased Gas | Corporate
Long-term Purchase Commitment [Line Items]
Revenue from Sales	27	31
Costs and Expenses	27	30
Utilized Firm Transportation Expense | Corporate
Long-term Purchase Commitment [Line Items]
Costs and Expenses	15	5
Unutilized Firm Transportation Expense | Corporate
Long-term Purchase Commitment [Line Items]
Costs and Expenses	0	1
Cost of Purchased Gas and Related Expense, Total
Long-term Purchase Commitment [Line Items]
Costs and Expenses	87	57
Cost of Purchased Gas and Related Expense, Total | Corporate
Long-term Purchase Commitment [Line Items]
Costs and Expenses	$ 42	$ 36