Use of Collateral (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Financial Instruments Owned and Pledged as Collateral [Line Items]
Margin Deposit Assets	[1]	$ 343	$ 221
Natural gas and power prepayments		31	23
Total margin deposits and natural gas and power prepayments with our counterparties	[2]	374	244
Letters of credit issued		1,166	885
First priority liens under power and natural gas agreements		92	102
First priority liens under interest rate hedging instruments		10	31
Letters of Credit Issued and First Priority Liens Under Power Natural Gas And Interest Rate Hedging Instruments		1,268	1,018
Margin deposits posted with us by our counterparties	[1],[3]	52	4
Letters of credit posted with us by our counterparties		27	30
Total margin deposits and letters of credit posted with us by our counterparties		79	34
Prepaid Expenses and Other Current Assets [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Total margin deposits and natural gas and power prepayments with our counterparties		286	171
Other Assets [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Total margin deposits and natural gas and power prepayments with our counterparties		9	9
Current and Non-current Derivative Assets and Liabilities [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Total margin deposits and natural gas and power prepayments with our counterparties		79	64
Margin deposits posted with us by our counterparties		32	2
Other Current Liabilities [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Margin deposits posted with us by our counterparties		$ 20	$ 2

[1] We offset fair value amounts recognized for derivative instruments executed with the same counterparty under a master netting arrangement for financial statement presentation; therefore, amounts recognized for the right to reclaim, or the obligation to return, cash collateral are presented net with the corresponding derivative instrument fair values. See Note 10 for further discussion of our derivative instruments subject to master netting arrangements.

[2] At December 31, 2018 and 2017, $79 million and $64 million, respectively, were included in current and long-term derivative assets and liabilities, $286 million and $171 million, respectively, were included in margin deposits and other prepaid expense and $9 million and $9 million, respectively, were included in other assets on our Consolidated Balance Sheets.

[3] At December 31, 2018 and 2017, $32 million and $2 million, respectively, were included in current and long-term derivative assets and liabilities and $20 million and $2 million, respectively, were included in other current liabilities on our Consolidated Balance Sheets.