Advances (Borrowers Holding Five Percent or more of Total Advances) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Advances [Line Items]
Advances	$ 58,282,314	$ 53,943,961
Advances [Member]
Advances [Line Items]
Advances	42,285,000	38,304,000
Concentration Risk, Percentage, Five Percent or More Of Principal Balance	73.00%	71.00%
JPMorgan Chase Bank National Association [Member] | Advances [Member]
Advances [Line Items]
Advances	37,700,000	26,000,000
Concentration Risk, Percentage, Five Percent or More Of Principal Balance	65.00%	48.00%
U.S. Bank, N.A. [Member] | Advances [Member]
Advances [Line Items]
Advances	4,585,000	4,586,000
Concentration Risk, Percentage, Five Percent or More Of Principal Balance	8.00%	8.00%
Fifth Third Bank [Member] | Advances [Member]
Advances [Line Items]
Advances		4,732,000
Concentration Risk, Percentage, Five Percent or More Of Principal Balance		9.00%
PNC Bank, N.A. [Member] | Advances [Member]
Advances [Line Items]
Advances		$ 2,986,000	[1]
Concentration Risk, Percentage, Five Percent or More Of Principal Balance		6.00%	[1]

[1] Former member.