Commitments and Contingencies (Commitments to Extend/Purchase) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commitments to purchase	$ 9,231		$ 9,231		$ 10,058
First Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased during period	3,600	$ 1,600	10,400	$ 6,500
Commitments to purchase	2,672		2,672		1,917
HELOCs [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased during period	112	$ 122	325	$ 362
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	$ 6,559		$ 6,559		$ 8,141