Allowance for Credit Losses (Credit Quality Indicators) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	$ 123	$ 117
Total Current Loans	9,260	7,759
Total Recorded Investment of Mortgage Loans	9,383	7,876
Mortgage Loans in Process of Foreclosure, Amount	$ 6	$ 10
Serious Delinquency Rate	0.00%	0.00%
Past Due 90 Days or More and Still Accruing Interest	$ 7	$ 8
Non-Accrual Mortgage Loans	31	36
Financial Asset, 30 to 59 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	77	66
Financial Asset, 60 to 89 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	19	19
Financing Receivables, 90 to 179 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	14	15
Financing Receivables, Greater than 180 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	13	17
Conventional Mortgage Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	91	87
Total Current Loans	8,783	7,273
Total Recorded Investment of Mortgage Loans	8,874	7,360
Mortgage Loans in Process of Foreclosure, Amount	$ 5	$ 8
Serious Delinquency Rate	0.00%	0.00%
Past Due 90 Days or More and Still Accruing Interest	$ 0	$ 0
Non-Accrual Mortgage Loans	31	36
Conventional Mortgage Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	57	49
Conventional Mortgage Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	14	14
Conventional Mortgage Loans [Member] | Financing Receivables, 90 to 179 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	10	11
Conventional Mortgage Loans [Member] | Financing Receivables, Greater than 180 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	10	13
Government Mortgage Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	32	30
Total Current Loans	477	486
Total Recorded Investment of Mortgage Loans	509	516
Mortgage Loans in Process of Foreclosure, Amount	$ 1	$ 2
Serious Delinquency Rate	1.00%	2.00%
Past Due 90 Days or More and Still Accruing Interest	$ 7	$ 8
Non-Accrual Mortgage Loans	0	0
Government Mortgage Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	20	17
Government Mortgage Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	5	5
Government Mortgage Loans [Member] | Financing Receivables, 90 to 179 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	4	4
Government Mortgage Loans [Member] | Financing Receivables, Greater than 180 Days Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Past Due Loans	$ 3	$ 4