Mortgage Loans Held for Portfolio by Collateral/Guarantee Type (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Loans and Leases Receivable Disclosure [Line Items]
Total par value	$ 4,462,014	$ 4,432,458
Conventional Mortgage Loan [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total par value	4,180,145	4,140,255
US Government Agency Insured Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total par value	$ 281,869	$ 292,203