Mortgage Loans Credit Quality Indicators - MF (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in loans	$ 3,237,616	$ 3,189,975
Multifamily [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in loans	285,954	267,001
Multifamily [Member] | Non-Classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in loans	280,952	263,416
Multifamily [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in loans	$ 5,002	$ 3,585