Mortgage Loans Held for Portfolio (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 7,041,859		$ 7,366,141
Loans and Leases Receivable, Unamortized Premiums	171,975		164,243
Loans and Leases Receivable, Unamortized Discounts	(3,723)		(3,605)
Loans and Leases Receivable, Hedging Basis Adjustment	17,202	[1]	21,240	[1]
Loans and Leases Receivable, Gross, Consumer, Mortgage	7,227,313		7,548,019
Fixed rates medium-term single-family mortgages [Member]
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	1,601,982	[2]	1,695,018	[2]
Fixed rates Long-term single-family mortgages [Member]
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	5,439,877		5,671,123
Conventional Loan [Member]
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	6,123,704		6,382,835
Federal Housing Administration Loan [Member]
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 918,155		$ 983,306

[1] Represents the unamortized balance of the mortgage purchase commitments' market values at the time of settlement. The market value of the commitment is included in the basis of the mortgage loan and amortized accordingly.

[2] Medium-term is defined as a term of 15 years or less.