Debt and Financing Arrangement (Notes)	6 Months Ended
Jun. 30, 2018
Debt Disclosure [Abstract]
Debt and Financing Arrangement	DEBT AND FINANCING ARRANGEMENT Credit Facility We, along with WPZ and Transcontinental Gas Pipe Line Company, LLC (Transco), are party to a credit facility with aggregate commitments available of $3.5 billion, with up to an additional $500 million increase in aggregate commitments available under certain circumstances. Total letter of credit capacity available to WPZ under this credit facility is $1.125 billion. We are able to borrow up to $500 million under this credit facility to the extent not otherwise utilized by the other co-borrowers. At June 30, 2018, no letters of credit have been issued and no loans were outstanding under the credit facility. WPZ participates in a commercial paper program, and WPZ management considers amounts outstanding under this program to be a reduction of available capacity under the credit facility. The program allows a maximum outstanding amount at any time of $3 billion of unsecured commercial paper notes. At June 30, 2018, WPZ had no commercial paper outstanding under the commercial paper program. During July 2018, in anticipation of the WPZ Merger closing, Williams entered into a new $4.5 billion unsecured revolving credit agreement (Credit Agreement) that includes us and Transco as co-borrowers. The full amount of the credit facility will be available to Williams to the extent not utilized by us or Transco. Both Transco and Northwest will have access of up to $500 million under this Credit Agreement. The Credit Agreement will become effective upon closing of the WPZ Merger and will be initially available for five years from the Credit Agreement effective date. Issuance and Retirement of Long-Term Debt On April 3, 2017, we issued $250 million of 4.0 percent senior unsecured notes due 2027 to investors in a private debt placement. We used the net proceeds to retire $185 million of 5.95 percent senior unsecured notes that matured on April 15, 2017, and for general corporate purposes. As part of the issuance, we entered into a registration rights agreement with the initial purchasers of the unsecured notes. Under the terms of the agreement, we were obligated to file and consummate a registration statement for an offer to exchange the notes for a new issue of substantially identical notes registered under the Securities Act of 1933, as amended, within 365 days from closing and to use commercially reasonable efforts to complete the exchange offer. We have filed the registration statement, which became effective in January 2018. The exchange offer was completed on March 1, 2018. On June 15, 2018, we repaid the $250 million, 6.05 percent senior unsecured notes that matured on that date, utilizing WPZ's cash management program by calling the advances due to us by WPZ combined with a short-term note payable to WPZ.

DEBT AND FINANCING ARRANGEMENT

Credit Facility

We, along with WPZ and Transcontinental Gas Pipe Line Company, LLC (Transco), are party to a credit facility with aggregate commitments available of $3.5 billion, with up to an additional $500 million increase in aggregate commitments available under certain circumstances. Total letter of credit capacity available to WPZ under this credit facility is $1.125 billion. We are able to borrow up to $500 million under this credit facility to the extent not otherwise utilized by the other co-borrowers. At June 30, 2018, no letters of credit have been issued and no loans were outstanding under the credit facility.

WPZ participates in a commercial paper program, and WPZ management considers amounts outstanding under this program to be a reduction of available capacity under the credit facility. The program allows a maximum outstanding amount at any time of $3 billion of unsecured commercial paper notes. At June 30, 2018, WPZ had no commercial paper outstanding under the commercial paper program.

During July 2018, in anticipation of the WPZ Merger closing, Williams entered into a new $4.5 billion unsecured revolving credit agreement (Credit Agreement) that includes us and Transco as co-borrowers. The full amount of the credit facility will be available to Williams to the extent not utilized by us or Transco. Both Transco and Northwest will have access of up to $500 million under this Credit Agreement. The Credit Agreement will become effective upon closing of the WPZ Merger and will be initially available for five years from the Credit Agreement effective date.

Issuance and Retirement of Long-Term Debt

On April 3, 2017, we issued $250 million of 4.0 percent senior unsecured notes due 2027 to investors in a private debt placement. We used the net proceeds to retire $185 million of 5.95 percent senior unsecured notes that matured on April 15, 2017, and for general corporate purposes. As part of the issuance, we entered into a registration rights agreement with the initial purchasers of the unsecured notes. Under the terms of the agreement, we were obligated to file and consummate a registration statement for an offer to exchange the notes for a new issue of substantially identical notes registered under the Securities Act of 1933, as amended, within 365 days from closing and to use commercially reasonable efforts to complete the exchange offer. We have filed the registration statement, which became effective in January 2018. The exchange offer was completed on March 1, 2018.

On June 15, 2018, we repaid the $250 million, 6.05 percent senior unsecured notes that matured on that date, utilizing WPZ's cash management program by calling the advances due to us by WPZ combined with a short-term note payable to WPZ.