Derivative Instruments (Details Textuals) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($) agreement	Jun. 30, 2018 USD ($) agreement	Dec. 31, 2017 USD ($) agreement
Cash Flow Hedges
Derivative [Line Items]
Unamortized losses associated with prior interest rate hedges	$ 18	$ 18	$ 22
Gains (losses) to be amortized to interest expenses during next twelve months		$ (4)
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	$ 2	$ 3
NUGs
Derivative [Line Items]
Liability position	65	65	$ 79
Settlements	$ 9	16
Unrealized gain (loss)		$ (2)