Derivative Instruments (Details Textuals) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) agreement	Dec. 31, 2017 USD ($) agreement
Cash Flow Hedges
Derivative [Line Items]
Unamortized losses associated with prior interest rate hedges	$ 20	$ 22
Gains (losses) to be amortized to interest expenses during next twelve months	$ (6)
Number of outstanding commodity or interest rate derivatives | agreement	0	0
Fair Value Hedging
Derivative [Line Items]
Number of fixed-for-floating interest rate swap agreements outstanding | agreement		0
Gains included in long-term debt associated with prior fixed-for-floating interest rate swap agreements	$ 3	$ 3
NUGs
Derivative [Line Items]
Liability position	74	$ 79
Settlements	7
Unrealized gain (loss)	$ (2)