Fair Value Level 4 (Details) - Fair Value of Debt - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt	$ 3,777	$ 3,542
Long-term Debt, fair value	3,236	3,143
Fair Value Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, fair value	0	0
Fair Value Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, fair value	3,205	3,134
Fair Value Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, fair value	$ 31	$ 9