Revenue - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Contract assets	$ 12		$ 12		$ 12
TCC contract liabilities	6		6		10
Revenue recognized	6	$ 7	16	$ 16
Accounts receivable related to contracts with customers	988		988		1,050
Unbilled revenues	266		266		$ 345
Revenue, remaining performance obligation, amount	245		245
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-10-01
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, amount	24		24
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, amount	67		67
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, amount	42		42
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, amount	25		25
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, amount	17		17
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, amount	14		14
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, amount	$ 80		$ 80
Renewables
Disaggregation of Revenue [Line Items]
Capitalized contract cost amortization term			15 years