Debt (Recourse Debt Net Principal Amounts Due Over Five Years) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Details [Line Items]
Recourse Debt Total	$ 4,630	$ 4,671
Unamortized (discounts)/premiums & debt issuance (costs)
Debt Details [Line Items]
Recourse Debt Total	(40)
Parent Company [Member]
Debt Details [Line Items]
Recourse Debt Total	4,630
Parent Company [Member] | 2018
Debt Details [Line Items]
Recourse Debt Total	5
Parent Company [Member] | 2019
Debt Details [Line Items]
Recourse Debt Total	5
Parent Company [Member] | 2020
Debt Details [Line Items]
Recourse Debt Total	234
Parent Company [Member] | 2021
Debt Details [Line Items]
Recourse Debt Total	902
Parent Company [Member] | 2022
Debt Details [Line Items]
Recourse Debt Total	500
Parent Company [Member] | Thereafter
Debt Details [Line Items]
Recourse Debt Total	3,024
Parent Company [Member] | Unamortized (discounts)/premiums & debt issuance (costs)
Debt Details [Line Items]
Recourse Debt Total	$ (40)	$ (45)