Debt Senior Unsecured Notes (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt	$ 11,741	$ 12,046
Senior Unsecured Notes 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt	1,238	1,237
Senior Unsecured Notes 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt	643	643
Senior Unsecured Notes 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	1,533	1,532
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 3,414	$ 3,412