Concentrations of Credit Risk Geographic Concentration (Details) (Guaranty Book of Business [Member])	Dec. 31, 2012		Dec. 31, 2011
Multifamily [Member]
Concentration Risk [Line Items]
Percentage of loans with detailed loan level information	99.00%		99.00%
Percentage of book of business	100.00%	[1],[2]	100.00%	[1],[2]
Multifamily [Member] | Midwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	8.00%	[1],[2]	8.00%	[1],[2]
Multifamily [Member] | Northeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	21.00%	[1],[2]	21.00%	[1],[2]
Multifamily [Member] | Southeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	21.00%	[1],[2]	20.00%	[1],[2]
Multifamily [Member] | Southwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	18.00%	[1],[2]	17.00%	[1],[2]
Multifamily [Member] | West [Member]
Concentration Risk [Line Items]
Percentage of book of business	32.00%	[1],[2]	34.00%	[1],[2]
Conventional Mortgage [Member] | Single-Family [Member]
Concentration Risk [Line Items]
Percentage of loans with detailed loan level information	99.00%		99.00%
Percentage of book of business	100.00%	[1],[3]	100.00%	[1],[3]
Conventional Mortgage [Member] | Single-Family [Member] | Midwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	15.00%	[1],[3]	15.00%	[1],[3]
Conventional Mortgage [Member] | Single-Family [Member] | Northeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	19.00%	[1],[3]	19.00%	[1],[3]
Conventional Mortgage [Member] | Single-Family [Member] | Southeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	23.00%	[1],[3]	24.00%	[1],[3]
Conventional Mortgage [Member] | Single-Family [Member] | Southwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	16.00%	[1],[3]	15.00%	[1],[3]
Conventional Mortgage [Member] | Single-Family [Member] | West [Member]
Concentration Risk [Line Items]
Percentage of book of business	27.00%	[1],[3]	27.00%	[1],[3]
Minimum [Member] | Conventional Mortgage [Member] | Single-Family [Member]
Concentration Risk [Line Items]
Percentage of loans with detailed loan level information	99.00%		99.00%

[1] Midwest includes IL, IN, IA, MI, MN, NE, ND, OH, SD, WI; Northeast includes CT, DE, ME, MA, NH, NJ, NY, PA, PR, RI, VT, VI; Southeast includes AL, DC, FL, GA, KY, MD, MS, NC, SC, TN, VA, WV; Southwest includes AZ, AR, CO, KS, LA, MO, NM, OK, TX, UT; West includes AK, CA, GU, HI, ID, MT, NV, OR, WA and WY.

[2] Consists of the portion of our multifamily guaranty book of business for which we have detailed loan level information, which constituted 99% of our total multifamily guaranty book of business as of December 31, 2012 and 2011.

[3] Consists of the portion of our single-family conventional guaranty book of business for which we have detailed loan level information, which constituted over 99% of our total single-family conventional guaranty book of business as of December 31, 2012 and 2011.