Related Party Transactions and Major Customers (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Dividend to parent		$ 32	$ 32
Revenues from major customers		755	638
Affiliates of NRG Energy, Inc. [Member]
Related Party Transaction [Line Items]
Revenues from major customers		161	152
Affiliates of Vistra Energy Corp. [Member]
Related Party Transaction [Line Items]
Revenues from major customers		54	47
Operation And Maintenance Expense [Member] | CenterPoint Energy [Member]
Related Party Transaction [Line Items]
Corporate service charges		44	42
Operation And Maintenance Expense [Member] | CERC Corp [Member]
Related Party Transaction [Line Items]
Corporate service charges		3	2
Other revenues from transactions with related party		$ (5)	$ (3)
Investments [Member]
Related Party Transaction [Line Items]
Weighted average interest rate of money pool investments		2.27%		1.90%
Accounts and notes receivable (payable) - affiliate companies [Member]
Related Party Transaction [Line Items]
Money pool investments (borrowings) (1)	[1]	$ 133		$ (60)

[1] Included in Accounts and notes receivable (payable)–affiliated companies in the Condensed Consolidated Balance Sheets.