Revenue - Schedule of Aggregate Transaction Price Allocated to Unsatisfied Performance Obligations and Expected Time to Recognize Revenue (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Utility Revenue [Line Items]
2019	$ 25
2020	29
2021	15
2022	7
2023	4
Thereafter	5
Total	85
Capacity and Carbon-free Energy Sale Contracts [Member]
Utility Revenue [Line Items]
2020	14
2021	8
2022	3
2023	3
Thereafter	4
Total	32
Renewable Energy Credit Sale Contracts [Member]
Utility Revenue [Line Items]
2019	25
2020	15
2021	7
2022	4
2023	1
Thereafter	1
Total	$ 53