Revenue from contracts with customers transaction price allocated to remaining performance obligations (Detail) - Electricity and Natural Gas [Member] $ in Millions	Mar. 31, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 5,359
Less than 12 Months [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 1,007
Performance obligations expected to be satisfied, period	1 year
Greater than 12 Months [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 4,352
Performance obligations expected to be satisfied, period