Mortgage Loans Credit Quality Indicators - MF (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in loans	$ 3,273,795.0	$ 3,255,168.0
Multifamily [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in loans	313,545.0	295,664.0
Multifamily [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in loans	0.5	1.0
Multifamily [Member] | Non-Classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in loans	306,257.0	289,231.0
Multifamily [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment in loans	$ 7,288.0	$ 6,433.0