Commitments and Contingencies (Commitments to Extend/Purchase) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commitments to purchase	$ 10,368	$ 8,911
Home Equity Loans and Lines of Credit, Pledged Asset Lines and Other Lines of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commitments to extend credit related to unused HELOCs, PALs, and other lines of credit	10,060	8,445
First Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commitments to purchase	$ 308	$ 466