Subsequent Event - Additional Information (Detail) - Subsequent Event [Member] $ in Millions	Oct. 16, 2015 USD ($) a
Two-acre Parcel of Unimproved Land [Member]
Subsequent Event [Line Items]
Area of land | a	2
Carrying value of land available for sale	$ 5
Purchase price of land available for sale	$ 14
Two-acre Parcel of Unimproved Land [Member] | Potomac Electric Power Co [Member]
Subsequent Event [Line Items]
Area of land | a	2
Carrying value of land available for sale	$ 5
Purchase price of land available for sale	$ 14
1.8-acre Land Parcel [Member]
Subsequent Event [Line Items]
Area of land | a	1.8
Carrying value of land	$ 4
Purchase price of land	$ 13
1.8-acre Land Parcel [Member] | Potomac Electric Power Co [Member]
Subsequent Event [Line Items]
Area of land | a	1.8
Carrying value of land	$ 4
Purchase price of land	$ 13