Commitments and Contingencies (Commitments to Extend/Purchase) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commitments to purchase	$ 12,803		$ 12,274
First Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased during period	2,200	$ 411
Commitments to purchase	3,095		1,521
HELOCs [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased during period	107	$ 62
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	$ 9,708		$ 10,753