Risk Management Credit Risks (Details) - Energy commodity derivative contracts(a)	Sep. 30, 2017 USD ($) notches	Dec. 31, 2016 USD ($)
Credit Derivatives [Line Items]
Letters of Credit Outstanding, Amount	$ 0	$ 0
Initial Margin Requirements	18,000,000
Variation Margin Requirements	$ 5,000,000
One Notch Downgrade to Credit Rating | notches	1
Two Notch Downgrade to Credit Rating | notches	2
Cash collateral held
Credit Derivatives [Line Items]
Collateral posted(b)	$ 13,000,000	$ 37,000,000
One notch credit downgrade [Member]
Credit Derivatives [Line Items]
Additional Collateral, Aggregate Fair Value	4,000,000
Two notch credit downgrade [Member]
Credit Derivatives [Line Items]
Additional Collateral, Aggregate Fair Value	$ 0