Revenue from contracts with customers transaction price allocated to remaining performance obligations (Detail) $ in Millions	Jun. 30, 2019 USD ($)
Electricity and Natural Gas [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 6,346
Electricity and Natural Gas [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 935
Performance obligations expected to be satisfied, period	1 year
Electricity and Natural Gas [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 5,411
Performance obligations expected to be satisfied, period
Other Sales and Service Contracts [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 3,327
Other Sales and Service Contracts [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 1,211
Performance obligations expected to be satisfied, period	1 year
Other Sales and Service Contracts [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 2,116
Performance obligations expected to be satisfied, period