DEBT AND CREDIT FACILITIES - Schedule of debt and credit facilities (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2020	Dec. 31, 2019	Jun. 01, 2020
Debt Instrument [Line Items]
Debt, gross	$ 2,119	$ 2,012
Less: unamortized debt issuance costs and debt discount	7	9
Current portion of long-term debt	350	123
Debt, net	1,762	1,880
Revolving credit facility \| Senior Credit Facility due 2021
Debt Instrument [Line Items]
Debt, gross	$ 0	$ 0
Weighted average interest rate	0.00%	0.00%
Term loan \| 2013 Term Loan Facility due 2022
Debt Instrument [Line Items]
Debt, gross	$ 450	$ 450
Weighted average interest rate	2.34%	3.52%
Unsecured debt \| 4.65% Unsecured Senior Notes due 2021
Debt Instrument [Line Items]
Stated interest rate	4.65%
Debt, gross	$ 350	$ 350
Weighted average interest rate	4.65%	4.65%
Unsecured debt \| 4.375% Unsecured Senior Notes due 2025
Debt Instrument [Line Items]
Stated interest rate	4.375%
Debt, gross	$ 350	$ 350
Weighted average interest rate	4.375%	4.375%
Unsecured debt \| 3.90% Unsecured Senior Notes due 2027
Debt Instrument [Line Items]
Stated interest rate	3.90%
Debt, gross	$ 500	$ 500
Weighted average interest rate	3.90%	3.90%
GTN \| Secured debt \| 3.12% Series A Senior Notes due 2030
Debt Instrument [Line Items]
Stated interest rate	3.12%		3.12%
Debt, gross	$ 175	$ 0
Weighted average interest rate	3.12%	0.00%
GTN \| Unsecured debt \| 5.29% Unsecured Senior Notes due 2020
Debt Instrument [Line Items]
Stated interest rate	5.29%	5.29%	5.29%
Debt, gross	$ 0	$ 100
Weighted average interest rate	0.00%	5.29%
GTN \| Unsecured debt \| 5.69% Unsecured Senior Notes due 2035
Debt Instrument [Line Items]
Stated interest rate	5.69%
Debt, gross	$ 150	$ 150
Weighted average interest rate	5.69%	5.69%
PNGTS \| Secured debt \| Revolving Credit Facility due 2023
Debt Instrument [Line Items]
Debt, gross	$ 71	$ 39
Weighted average interest rate	2.36%	3.47%
Tuscarora \| Unsecured debt \| Unsecured Term Loan due 2021
Debt Instrument [Line Items]
Debt, gross	$ 23	$ 23
Weighted average interest rate	2.22%	3.39%
North Baja \| Unsecured debt \| Unsecured Term Loan due 2021
Debt Instrument [Line Items]
Debt, gross	$ 50	$ 50
Weighted average interest rate	2.17%	3.34%

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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- Definition

The average effective interest rate during the reporting period.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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