Derivative Instruments and Hedging - Net Notional Volume (Detail) gal in Millions, dth in Millions, MWh in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020 gal dth MWh	Dec. 31, 2019 gal dth MWh
Networks | Wholesale Electricity Contract | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, nonmonetary notional amount | MWh	4.7	5.1
Networks | Natural Gas Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, nonmonetary notional amount	7.7	8.5
Networks | Fleet Fuel Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, nonmonetary notional amount | gal	2.3	2.2
Renewables and Gas Activities | Long | Basis Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, nonmonetary notional amount	41.0	42.0
Renewables and Gas Activities | Short | Basis Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, nonmonetary notional amount	2.0	1.0
Renewables and Gas Activities | Wholesale Electricity Contract | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, nonmonetary notional amount, energy measure (MWh) | MWh	4.0	4.0
Renewables and Gas Activities | Wholesale Electricity Contract | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, nonmonetary notional amount, energy measure (MWh) | MWh	9.0	9.0
Renewables and Gas Activities | Natural Gas and Other fuel Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, nonmonetary notional amount	29.0	29.0
Renewables and Gas Activities | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, nonmonetary notional amount	11.0	10.0