Commitments and Contingencies (Commitments to Extend/Purchase) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commitments to purchase			$ 13,210		$ 11,314
First Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased during period	$ 842	$ 491	2,000	$ 1,600
Commitments to purchase			1,976		268
HELOCs [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased during period	52	$ 104	180	$ 311
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	$ 11,234		$ 11,234		$ 11,046