Securities Sold Under Agreements To Repurchase (Narrative) (Details) (USD $)	Sep. 30, 2011 years	May 31, 2011	Dec. 31, 2010	Sep. 30, 2011 Short-Term Securities [Member]	Mar. 31, 2011 Long-term Securities [Member] securities	Sep. 30, 2011 Minimum [Member]	Sep. 30, 2011 Maximum [Member]
Securities Sold Under Agreements To Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 1,407,500,000		$ 1,561,000,000	$ 7,500,000	$ 100,000,000
Weighted average rate of securities sold under agreements to repurchase	4.12%		4.18%	0.80%
Number of long-tem securities sold					2
Long-term securities, weighted average interest rate					4.77%
Prepaid security sold under repurchase agreement		50,000,000
Rate of prepaid security		4.83%
Prepayment penalty security sold under agreements to repurchase		1,700,000
Total floating-to-fixed rate agreements	750,000,000
Number of floating-to-fixed rate agreements	14
Initial floating rate period on floating-to-fixed rate agreements, months, minimum	6
Initial floating rate period on floating-to-fixed rate agreements, years, maximum	1
Maximum initial floating rate, 3-month LIBOR minus 100 basis point	100
Minimum initial floating rate, 3-month LIBOR minus 340 basis point	340
Current lowest rate-long term initial floating-to-fix security sold under agreements to repurchase						4.29%	5.07%
Number of short-term securities sold under agreement				1
Total floating-to-fixed rate agreements	650,000,000
Number of fixed-to-floating securities sold under agreements to repurchase	13
Minimum initial fixed rate	1.00%
Maximum initial fixed rate	3.50%
Initial fixed rate period (in months)						6	18
Remainder of long-term securities, years	7
Maximum interest rate						3.25%	3.75%
Minimum interest rate	0.00%
Mortgage-backed securities with fair value	$ 1,500,000,000		$ 1,700,000,000