Regulatory Matters - Summary of Utilities Rate Plans (O&R New York-Gas) (Details) - USD ($)	12 Months Ended					36 Months Ended	62 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2018	Dec. 31, 2014
Public Utilities, General Disclosures [Line Items]
Cost reconciliations, deferred net regulatory assets	$ 822,000,000	$ 836,000,000
O&R | Gas
Public Utilities, General Disclosures [Line Items]
Deferred revenues	6,200,000	(800,000)	$ (100,000)	$ 700,000	$ 4,700,000		$ (100,000)
Negative revenue adjustments	0	0	0	0	0
Cost reconciliations, deferred net regulatory assets	6,600,000	4,500,000	8,300,000	8,300,000	700,000		$ 8,300,000
Net utility plant reconciliations		$ 0	0	0	700,000
Weighted average cost of capital (after-tax) (percent)							8.49%
Authorized return on common equity (percent)							10.40%
Earnings sharing, threshold limit	$ 4,000,000		$ 0	$ 0	$ 0
Cost of long-term debt (percent)			6.81%				6.81%
Common equity ratio (percent)							48.00%
O&R | Gas | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Authorized return on common equity (percent)						9.00%
Common equity ratio (percent)						48.00%
O&R | Gas | Scenario, Forecast | Property Tax and Interest Rate Reconciliations
Public Utilities, General Disclosures [Line Items]
Regulatory assets not recoverable						$ 14,000,000
O&R | Gas | Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes							$ 9,000,000
Amortization to income of net regulatory (assets) and liabilities							(2,000,000)
Average rate base							$ 280,000,000
Earnings sharing (percentage)			11.40%				11.40%
O&R | Gas | Year 1 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							$ 1,400,000.0
O&R | Gas | Year 1 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes						16,400,000
Amortization to income of net regulatory (assets) and liabilities						(1,700,000)
Average rate base						$ 366,000,000
Weighted average cost of capital (after-tax) (percent)						7.10%
Earnings sharing (percentage)						9.60%
Cost of long-term debt (percent)						5.42%
Rate exclusion amount with balance below regulatory threshold						$ 500,000
O&R | Gas | Year 1 | Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)						3,700,000.0
Net utility plant reconciliations						492,000,000
O&R | Gas | Year 2
Public Utilities, General Disclosures [Line Items]
Base rate changes							9,000,000
Amortization to income of net regulatory (assets) and liabilities							(2,000,000)
Average rate base							$ 296,000,000
Earnings sharing (percentage)			11.40%				11.40%
O&R | Gas | Year 2 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							$ 1,400,000.0
O&R | Gas | Year 2 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes						16,400,000
Amortization to income of net regulatory (assets) and liabilities						(2,100,000)
Average rate base						$ 391,000,000
Weighted average cost of capital (after-tax) (percent)						7.06%
Earnings sharing (percentage)						9.60%
Cost of long-term debt (percent)						5.35%
Rate exclusion amount with balance below regulatory threshold						$ 4,200,000
O&R | Gas | Year 2 | Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)						4,700,000.0
Net utility plant reconciliations						518,000,000
O&R | Gas | Year 3
Public Utilities, General Disclosures [Line Items]
Base rate changes							4,600,000
Base rate change through surcharge							4,300,000
Amortization to income of net regulatory (assets) and liabilities							(2,000,000)
Average rate base							$ 309,000,000
Earnings sharing (percentage)			11.40%				11.40%
O&R | Gas | Year 3 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							$ 1,400,000.0
O&R | Gas | Year 3 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes						5,800,000
Base rate change through surcharge						10,600,000
Amortization to income of net regulatory (assets) and liabilities						(2,500,000)
Average rate base						$ 417,000,000
Weighted average cost of capital (after-tax) (percent)						7.06%
Earnings sharing (percentage)						9.60%
Cost of long-term debt (percent)						5.35%
Rate exclusion amount with balance below regulatory threshold						$ 7,200,000
O&R | Gas | Year 3 | Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)						5,800,000.0
Net utility plant reconciliations						$ 546,000,000
O&R | Gas | Year 4
Public Utilities, General Disclosures [Line Items]
Base rate change through surcharge							0
O&R | Gas | Year 5
Public Utilities, General Disclosures [Line Items]
Base rate change through surcharge							$ 0