Derivative Instruments and Hedging Activities (Statement of Gains and Losses on Derivative Instruments Reported in Statement of Comprehensive Income) (Details) - Not designated as hedging instrument - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivative instruments, net, pretax	$ (1)	$ (3)	$ 5	$ (9)
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest rate derivative instruments	0	0	0	(2)
Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency derivative instruments	(1)	(3)	5	(7)
Other income | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest rate derivative instruments	0	0	0	(3)
Other income | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency derivative instruments	(1)	(3)	5	(7)
Gains on mortgage and automotive loans | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest rate derivative instruments	$ 0	$ 0	$ 0	$ 1