Short-term Borrowings and Long-term Debt (Details) - USD ($) $ in Millions					12 Months Ended
		Dec. 18, 2017	Nov. 01, 2017	Aug. 21, 2017	Dec. 31, 2017		Dec. 31, 2016
Debt Instrument [Line Items]
Short-term borrowings	[1]				$ 39		$ 35
Unamortized issuance costs, net					(12)		(10)
Unamortized discount (premium), net					(22)		(27)
Long-term debt, excluding current maturities					2,457		2,125
Long-term debt, current maturities	[1]				0		250
Total debt, current	[1]				39		285
Increase (decrease) in line of credit facility					300
Size of facility					900	[2]	600
Maturities in 2018					0
Maturities in 2019					0
Maturities in 2020					0
Maturities in 2021					593
Maturities in 2022					898
Inventory financing [Member]
Debt Instrument [Line Items]
Short-term borrowings	[1],[3]				39		35
Senior Notes [Member]
Debt Instrument [Line Items]
Extinguishment of debt, amount		$ 300	$ 250
Percentage debt redemption price		100.00%
Gain (loss) on extinguishment of debt		$ (5)
Aggregate principal amount				$ 300
Debt instrument interest rate		6.00%	6.125%	4.10%
Maturity date				Sep. 01, 2047
Senior Notes 4.10% to 6.625% due 2047 to 2041
Debt Instrument [Line Items]
Gross long-term debt, non-current					1,593		1,593
Gross long-term debt, current	[1]				0		250
Commercial Paper [Member]
Debt Instrument [Line Items]
Gross long-term debt, non-current	[4]				898		569
Gross long-term debt, current	[1],[4]				$ 0		0
Line of Credit [Member]
Debt Instrument [Line Items]
Percentage on limitation of debt to total capitalization under covenant (in hundredths)					65.00%
Ratio of indebtedness to net capital					0.404
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Loans					$ 0		0
Letter of Credit [Member]
Debt Instrument [Line Items]
Loans					1		4
Commercial Paper [Member]
Debt Instrument [Line Items]
Loans	[5]				$ 898		$ 569
Weighted average interest rate					1.72%		1.03%
London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate description	[6]				1.25%
Minimum [Member] | Senior Notes 4.10% to 6.625% due 2047 to 2041
Debt Instrument [Line Items]
Debt instrument interest rate					4.10%
Maximum [Member] | Senior Notes 4.10% to 6.625% due 2047 to 2041
Debt Instrument [Line Items]
Debt instrument interest rate					6.625%

[1] Includes amounts due or exchangeable within one year of the date noted.

[2] Amended on June 16, 2017 to increase the aggregate commitment size as noted above.

[3] NGD has AMAs associated with its utility distribution service in Arkansas, Louisiana, Mississippi, Oklahoma and Texas. The AMAs have varying terms, the longest of which expires in 2020. Pursuant to the provisions of the agreements, NGD sells natural gas and agrees to repurchase an equivalent amount of natural gas during the winter heating seasons at the same cost, plus a financing charge. These transactions are accounted for as an inventory financing.

[4] Classified as long-term debt because the termination date of the facility that backstops the commercial paper is more than one year from the date noted.

[5] Weighted average interest rate was approximately 1.72% and 1.03% as of December 31, 2017 and December 31, 2016, respectively.Execution Date Size ofFacility Draw Rate of LIBOR plus (2) Financial Covenant Limit on Debt for Borrowed Money to Capital Ratio Debt for Borrowed Money to Capital Ratio as of December 31, 2017 Termination Date (3) (in millions) March 3, 2016 $900(1)1.25% 65% 40.4% March 3, 2022

[6] Based on current credit ratings.