Fair Value of Financial Instruments and Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions		9 Months Ended
	May 24, 2017	Sep. 30, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other Assets		$ 74	$ 241
Fair value of debt		5,443	5,204
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of debt		$ 61	61
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value input, gas or power delivery period (in years)		2 years
Restricted Cash [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other Assets		$ 7	$ 5
RG&E [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of electric load obligations using contracts for a NYISO location		70.00%
RG&E [Member] | 3.10% First Mortgage Bonds due 2027 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate principal amount of bonds	$ 300
Interest rate on bonds	3.10%
Maturity year of bonds	2027