Regulatory - Additional Information (Detail) $ in Millions							3 Months Ended			12 Months Ended
	Jun. 28, 2019 USD ($) MW	Apr. 09, 2019 USD ($)	Aug. 20, 2018 USD ($)	Jun. 29, 2018 USD ($) MW	Dec. 12, 2017 USD ($) MW	Sep. 27, 2017 USD ($) MW	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2019 USD ($)	Oct. 31, 2013 USD ($)
Public Utilities General Disclosures [Line Items]
Federal statutory tax rate							4.50%			5.50%
Decrease in deferred income taxes offset to regulatory liability							$ 5
Provision for income taxes							16	$ 16
Amount refundable on settlement		$ 12
Peoples Gas System [Member]
Public Utilities General Disclosures [Line Items]
Reduction in annual depreciation rates							10
Reduction in annual base rates							12
Hurricane Irma [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs									$ 102
Hurricane Dorian [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration preparation costs										$ 8
Tax Reform and Storm Settlement [Member]
Public Utilities General Disclosures [Line Items]
Provision for income taxes							$ 103
Annually approved lowering base rates, Amount			$ 103
Solar Project Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Settlement agreement, extended terms						four years through December 31, 2021
Settlement agreement, approval date						Nov. 06, 2017
Solar energy capacity | MW						600
Solar generation capacity investments						$ 850
Solar Project Cost Recovery [Member] | Effective September 1, 2018 [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW					145
Estimated annual revenue requirements					$ 24
Solar Project Cost Recovery [Member] | Effective January 1, 2019 [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW				260
Estimated annual revenue requirements				$ 46
Solar Project Cost Recovery [Member] | Effective January 1, 2020 [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW	149
Estimated annual revenue requirements	$ 26
Solar Project Cost Recovery [Member] | Effective Date January 1, 2021 [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW					50
Solar base rate adjustments					$ 10
Solar Project Cost Recovery [Member] | Minimum [Member]
Public Utilities General Disclosures [Line Items]
Solar project investment term						2017
Solar Project Cost Recovery [Member] | Maximum [Member]
Public Utilities General Disclosures [Line Items]
Solar project investment term						2021
Storm Restoration Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Minimum cost recovery period							12 months
Replenishment reserve for recovery of cost											$ 56