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Commitments and Contingencies (Commitments to Extend/Purchase) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Commitments to purchase $ 10,901   $ 10,058
First Mortgage [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Purchased during period 2,800 $ 2,200  
Commitments to purchase 3,222   1,917
HELOCs [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Purchased during period 99 $ 107  
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 $ 7,679   $ 8,141