Mortgage Loans Credit Quality Indicators - MF (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	$ 3,189,975	$ 3,095,816
Multifamily [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	267,001	231,881
Multifamily [Member] | Non-classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	263,416	228,749
Multifamily [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	3,585	3,129
Multifamily [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	0	3
Multifamily [Member] | Total classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	$ 3,585	$ 3,132