Allowance for Credit Losses (Recorded Investment, Unpaid Principal Balance and Related Allowance of Impaired Loans) (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Loans and Leases Receivable, Allowance		$ 0	$ 0
Conventional Mortgage Loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment, Allowance for credit losses		0	0
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment		0	0
Loans and Leases Receivable, Allowance		0	0
Financing Receivable, Individually Evaluated for Impairment		5	7
Financing Receivable, Collectively Evaluated for Impairment		3,329	3,077
Financing Receivable, before Allowance for Credit Loss	[1]	3,334	3,084
Impaired Financing Receivable, with No Related Allowance, Recorded Investment		5	7
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance		$ 5	$ 7

[1] The recorded investment in a loan is the unpaid principal balance of the loan, adjusted for accrued interest, net deferred loan fees or costs, unamortized premiums or discounts, and direct write-downs. The recorded investment is not net of any valuation allowance.