Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($) $ in Millions		Mar. 31, 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		7	8
Commodity Contracts | Recurring Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, total fair value		47	49
Liabilities, total fair value		26	38
Assets		(26)	(37)
Liabilities		(26)	(37)
Assets		21	12
Liabilities		0	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		5	5
Liabilities		18	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		42	44
Liabilities		8	7
Gas Imbalances | Recurring Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, total fair value	[1],[2]	13	14
Liabilities, total fair value	[2],[3]	10	11
Assets	[1],[2]	0	0
Liabilities	[2],[3]	0	0
Assets	[1],[2]	13	14
Liabilities	[2],[3]	10	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]	13	14
Liabilities	[2],[3]	$ 10	$ 11

[1] Gas imbalance assets exclude fuel reserves for under retained fuel due from shippers of $23 million and $21 million at March 31, 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 March 31, 2020 and December 31, 2019.

[3] Gas imbalance liabilities exclude fuel reserves for over retained fuel due to shippers of $7 million and $8 million at March 31, 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.