Commitments and Contingencies (Commitments to Extend/Purchase) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commitments to purchase			$ 11,333		$ 12,274
First Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased during period	$ 2,700	$ 702	4,900	$ 1,100
Commitments to purchase			1,992		1,521
HELOCs [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased during period	133	$ 66	240	$ 128
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	$ 9,341		$ 9,341		$ 10,753