Acquisitions, Investments and Dispositions - Additional Information (Details) $ in Millions		1 Months Ended						3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	Oct. 31, 2016 USD ($) MW	Sep. 30, 2016 USD ($)	Aug. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	May 31, 2016 USD ($)	Jan. 31, 2016 USD ($) MW	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) MW	Sep. 30, 2015 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Ownership interest, percentage														45.70%
Non-utility property			$ 1,127					$ 1,127		$ 1,127			$ 832
Other current assets			206					206		206			191
Gain on sale of business								104	$ 0	104	$ 0
Mountain Valley Pipeline LLC
Schedule of Equity Method Investments [Line Items]
Equity method ownership (percent)							12.50%
Payments to acquire equity method investments							$ 18
Equity method investments			41					41		41
Stagecoach Gas Services LLC
Schedule of Equity Method Investments [Line Items]
Equity method ownership (percent)					50.00%
Payments to acquire equity method investments					$ 945
Equity method investments			968					968		$ 968
Pilesgrove
Schedule of Equity Method Investments [Line Items]
Equity method ownership (percent)					50.00%
Impairment of investment					$ 8
Impairment of investment, net of tax					$ 5
New Jersey | Pilesgrove
Schedule of Equity Method Investments [Line Items]
Generating capacity | MW										18
Texas Solar 7 | Texas
Schedule of Equity Method Investments [Line Items]
Voting interest acquired (percentage)							100.00%
Generating capacity | MW							106
Payments to acquire business							$ 227
Non-utility construction work in progress							218
Other receivables							$ 9
Pilesgrove
Schedule of Equity Method Investments [Line Items]
Voting interest acquired (percentage)				50.00%
Net assets of project			48					48		$ 48
Non-utility property			45					45		45
Consideration transferred				$ 16
Bargain purchase gain				8
Bargain purchase gain, net of tax				5
Other current assets			3					3		3
Electric | Texas Solar 7 | Texas
Schedule of Equity Method Investments [Line Items]
Net assets of project			123					123		123
Scenario, Forecast | Panoche Valley | California
Schedule of Equity Method Investments [Line Items]
Voting interest acquired (percentage)		50.00%
Generating capacity | MW		240
Consideration transferred		$ 37
Scenario, Forecast | Gas | Minimum | Mountain Valley Pipeline LLC
Schedule of Equity Method Investments [Line Items]
Payments to acquire equity method investments												$ 3,000
Scenario, Forecast | Gas | Maximum | Mountain Valley Pipeline LLC
Schedule of Equity Method Investments [Line Items]
Payments to acquire equity method investments												$ 3,500
CET Electric
Schedule of Equity Method Investments [Line Items]
Ownership interest, percentage						45.70%	45.70%
CET Electric | NY Transco
Schedule of Equity Method Investments [Line Items]
Equity method investments			53					$ 53		$ 53
Purchase Price of TOTS Project Transfer | NY Transco | CET Electric
Schedule of Equity Method Investments [Line Items]
Related party transaction, amount of transaction						$ 122
Lease Payments | NY Transco | CET Electric
Schedule of Equity Method Investments [Line Items]
Expenses from transactions with related party						$ 8
Discontinued Operations, Held-for-sale | Pike
Schedule of Equity Method Investments [Line Items]
Impairment of long-lived assets to be disposed of	$ 5
Impairment of long-lived assets to be disposed of, net of taxes	$ 3
Assets held for sale													23
Liabilities held for sale													5
Discontinued Operations, Held-for-sale | Retail Electric Supply Business
Schedule of Equity Method Investments [Line Items]
Assets held for sale													134
Liabilities held for sale													$ 84
Discontinued Operations, Disposed of by Sale | Pike
Schedule of Equity Method Investments [Line Items]
Proceeds from divestiture of business				$ 15
Discontinued Operations, Disposed of by Sale | Pike | O&R
Schedule of Equity Method Investments [Line Items]
Period for transition services (up to)				18 months
Agreement to purchase and sell, term				3 years
Agreement to purchase and sell, term of extension option				3 years
Discontinued Operations, Disposed of by Sale | Retail Electric Supply Business
Schedule of Equity Method Investments [Line Items]
Proceeds from divestiture of business			235
Gain on sale of business			104
Gain on sale of business, after tax			47
Gain on sale of derivatives			65
Gain on sale of derivatives, after tax			42
Tax effect of sale, state tax related to change in apportionment of state income taxes			29
Tax effect of sale, state tax related to change in apportionment of state income taxes, net of federal tax			$ 19