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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
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	$ 2,825		$ 3,363
Delinquent Amount at End of Period on Loans Managed and Securitized or Asset-backed Financing Arrangement	15		20
Net Credit Losses During Period on Loans Managed or Securitized or Asset-backed Financing Arrangement	4	$ 4
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	1,658		1,964
Delinquent Amount at End of Period on Loans Managed and Securitized or Asset-backed Financing Arrangement	12		16
Net Credit Losses During Period on Loans Managed or Securitized or Asset-backed Financing Arrangement	3	3
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	1,658		1,964
Delinquent Amount at End of Period on Loans Managed and Securitized or Asset-backed Financing Arrangement	12		16
Net Credit Losses During Period on Loans Managed or Securitized or Asset-backed Financing Arrangement	3	3
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	1,167		1,399
Delinquent Amount at End of Period on Loans Managed and Securitized or Asset-backed Financing Arrangement	3		$ 4
Net Credit Losses During Period on Loans Managed or Securitized or Asset-backed Financing Arrangement	$ 1	$ 1