Revenue from contracts with customers transaction price allocated to remaining performance obligations (Detail) $ in Millions	Jun. 30, 2018 USD ($)
Electricity and Natural Gas [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 7,115
Electricity and Natural Gas [Member] | Less than 12 Months [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 1,160
Performance obligations expected to be satisfied, period	1 year
Electricity and Natural Gas [Member] | Greater than 12 Months [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 5,955
Performance obligations expected to be satisfied, period
Industrial and Commercial Products [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 3,029
Industrial and Commercial Products [Member] | Less than 12 Months [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 59
Performance obligations expected to be satisfied, period	1 year
Industrial and Commercial Products [Member] | Greater than 12 Months [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to remaining performance obligations	$ 2,970
Performance obligations expected to be satisfied, period