Derivative Instruments (Details Textuals)	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($) agreement	Sep. 30, 2018 USD ($) agreement	Dec. 31, 2017 USD ($) agreement
Cash Flow Hedges
Derivative [Line Items]
Unamortized losses associated with prior interest rate hedges	$ 16,000,000	$ 16,000,000	$ 22,000,000
Gains (losses) to be amortized to interest expenses during next twelve months		$ (3,000,000)
Number of outstanding commodity or interest rate derivatives | agreement	0	0	0
Fair Value Hedging
Derivative [Line Items]
Number of fixed-for-floating interest rate swap agreements outstanding | agreement	0	0	0
Gains included in long-term debt associated with prior fixed-for-floating interest rate swap agreements	$ 2,000,000	$ 2,000,000	$ 3,000,000
NUGs
Derivative [Line Items]
Liability position	52,000,000	52,000,000	79,000,000
Settlements	9,000,000	25,000,000
Unrealized gain (loss)	4,000,000	2,000,000
FTRs
Derivative [Line Items]
Settlements	0	(3,000,000)
Unrealized gain (loss)	9,000,000	9,000,000
Net asset position	$ 9,000,000	$ 9,000,000	$ 3,000,000