Securitizations and Variable Interest Entities (Schedule of Quantitative Information and Net Credit Losses about Securitized and Other Financial Assets Managed Together) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Quantitative Information about Securitized and Other Financial Assets Managed Together [Line Items]
Principal Amount Outstanding on Loans Managed and Securitized or Asset-backed Financing Arrangement	$ 3,363	$ 5,556
Delinquent Amount at End of Period on Loans Managed and Securitized or Asset-backed Financing Arrangement	20	19
Net Credit Losses During Period on Loans Managed or Securitized or Asset-backed Financing Arrangement	16	11
Off-Balance Sheet Loans [Member]
Quantitative Information about Securitized and Other Financial Assets Managed Together [Line Items]
Principal Amount Outstanding on Loans Managed and Securitized or Asset-backed Financing Arrangement	1,964	2,392
Delinquent Amount at End of Period on Loans Managed and Securitized or Asset-backed Financing Arrangement	16	13
Net Credit Losses During Period on Loans Managed or Securitized or Asset-backed Financing Arrangement	13	8
Off-Balance Sheet Loans [Member] | Consumer Portfolio Segment [Member] | Automobile Loan [Member]
Quantitative Information about Securitized and Other Financial Assets Managed Together [Line Items]
Principal Amount Outstanding on Loans Managed and Securitized or Asset-backed Financing Arrangement	1,964	2,392
Delinquent Amount at End of Period on Loans Managed and Securitized or Asset-backed Financing Arrangement	16	13
Net Credit Losses During Period on Loans Managed or Securitized or Asset-backed Financing Arrangement	13	8
Whole-Loan Transactions [Member]
Quantitative Information about Securitized and Other Financial Assets Managed Together [Line Items]
Principal Amount Outstanding on Loans Managed and Securitized or Asset-backed Financing Arrangement	1,399	3,164
Delinquent Amount at End of Period on Loans Managed and Securitized or Asset-backed Financing Arrangement	4	6
Net Credit Losses During Period on Loans Managed or Securitized or Asset-backed Financing Arrangement	$ 3	$ 3