Mortgage Loans Held for Portfolio (Details) (USD $) In Thousands, unless otherwise specified											9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Loans Receivable With Fixed Rates Of Interest Medium Term [Member]		Dec. 31, 2011 Loans Receivable With Fixed Rates Of Interest Medium Term [Member]		Sep. 30, 2012 Loans Receivable With Fixed Rates Of Interest Long Term [Member]		Dec. 31, 2011 Loans Receivable With Fixed Rates Of Interest Long Term [Member]		Sep. 30, 2012 Minimum [Member] Loans Receivable With Fixed Rates Of Interest Long Term [Member]	Sep. 30, 2012 Maximum [Member] Loans Receivable With Fixed Rates Of Interest Medium Term [Member]	Sep. 30, 2012 Due within one year	Dec. 31, 2011 Due within one year	Sep. 30, 2012 Due after one year through five years	Dec. 31, 2011 Due after one year through five years	Sep. 30, 2012 Due after five years	Dec. 31, 2011 Due after five years	Sep. 30, 2012 Government-Insured Mortgage Loans [Member]	Dec. 31, 2011 Government-Insured Mortgage Loans [Member]	Sep. 30, 2012 Conventional Loan [Member]	Dec. 31, 2011 Conventional Loan [Member]
Mortgage Loans on Real Estate [Line Items]
Par value of mortgage loans held for portfolio	$ 3,539,115	$ 3,853,254	$ 555,334	[1]	$ 601,104	[1]	$ 2,983,781	[1]	$ 3,252,150	[1]			$ 160	$ 99	$ 15,772	$ 5,006	$ 3,523,183	$ 3,848,149	$ 355,896	$ 345,346	$ 3,183,219	$ 3,507,908
Premiums	47,800	41,904
Discounts	(9,076)	(11,555)
Hedging adjustment	15,298	13,868
Total mortgage loans held for portfolio	$ 3,593,137	$ 3,897,471
Mortgage Loans on Real Estate, Original Contractual Terms											15 years	15 years

[1] Medium-term is defined as a term of 15 years or less. Long-term is defined as greater than 15 years.