Other-Than-Temporary Impairment (Narrative) (Details)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 Minimum [Member] area people	Dec. 31, 2011 Minimum [Member]	Sep. 30, 2012 Maximum [Member]	Sep. 30, 2012 MBS [Member] security	Sep. 30, 2012 MBS [Member] Minimum [Member] form	Sep. 30, 2012 Alternative Procedures [Member] MBS [Member]	Sep. 30, 2012 Cash Flow Analysis [Member] Private label residential MBS [Member] model	Sep. 30, 2012 Cash Flow Analysis, Model 1 [Member] projection
Other than Temporary Impairment, Disclosure [Line Items]
Projected Home Price Recovery, Number of Projections									100
Number of Third Party Models To Assess Recovery of Amortized Cost Basis of Securities								2
Marketable Securities, Par Balance, Percent							5.00%
Number of Securities with AA Rating					1
Core-Based Statistical Areas, Current Requirements, Number of Urban Areas		1
Core-Based Statistical Areas, Current Requirements, Urban Area, Population		10,000
Assumed Current to Trough Home Price Decline Rate			0.00%	4.00%
Projected House Price Decline Rate		1.00%		2.00%
Period Assumed For Housing Markets That Have Reached Trough	9 months
Number Of Forms Of Credit Enhancements						1