Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($) $ in Millions		Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Retained fuel due from shippers		$ 23	$ 21
Over retained fuel due from shippers		6	8
Commodity Contracts | Recurring Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, total fair value		38	49
Liabilities, total fair value		29	38
Asset netting adjustments		(28)	(37)
Liability netting adjustments		(28)	(37)
Assets		10	12
Liabilities		1	1
Commodity Contracts | Recurring Measurement | Quoted market prices in active market for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		6	5
Liabilities		23	31
Commodity Contracts | Recurring Measurement | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		32	44
Liabilities		6	7
Gas Imbalances | Recurring Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, total fair value	[1],[2]	15	14
Liabilities, total fair value	[2],[3]	13	11
Asset netting adjustments	[1],[2]	0	0
Liability netting adjustments	[2],[3]	0	0
Assets	[1],[2]	15	14
Liabilities	[2],[3]	13	11
Gas Imbalances | Recurring Measurement | Quoted market prices in active market for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	[1],[2]	0	0
Liabilities	[2],[3]	0	0
Gas Imbalances | Recurring Measurement | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	[1],[2]	15	14
Liabilities	[2],[3]	$ 13	$ 11

[1] Gas imbalance assets exclude fuel reserves for under retained fuel due from shippers of $23 million and $21 million at June 30, 2020 and December 31, 2019, respectively, which fuel reserves are based on the value of natural gas at the time the imbalance was created, and which are not subject to revaluation at fair market value.

[2] The Partnership uses the market approach to fair value its gas imbalance assets and liabilities at individual, or where appropriate an average of, current market indices applicable to the Partnership’s operations, not to exceed net realizable value. There were no netting adjustments as of June 30, 2020 and December 31, 2019.

[3] Gas imbalance liabilities exclude fuel reserves for over retained fuel due to shippers of $6 million and $8 million at June 30, 2020 and December 31, 2019, respectively, which fuel reserves are based on the value of natural gas at the time the imbalance was created, and which are not subject to revaluation at fair market value.