Debt Senior Unsecured Notes (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt	$ 11,368	$ 11,213
Senior Unsecured Notes 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt	1,240	1,239
Senior Unsecured Notes 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt	644	644
Senior Unsecured Notes 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	1,535	1,534
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 3,419	$ 3,417