Securities Sold Under Agreements To Repurchase (Narrative) (Details) (USD $)	Jun. 30, 2011	May 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Securities sold under agreement to repurchase, fair value	$ 1,411,500,000			$ 1,561,000,000
Weighted average rate of securities sold under agreements	4.11%			4.18%
Total securities sold under agreements to repurchase			100,000,000
Long-term debt, weighted average interest rate			4.77%
Prepaid security sold under repurchase agreement		50,000,000
Rate of prepaid security		4.83%
Prepayment penalty		1,700,000
Total floating-to-fixed rate	750,000,000
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
LIBOR minus 100 basis points, basis points	100
LIBOR minus 340 basis points, basis points	340
Total fixed to floating rate agreements	650,000,000
Minimum interest rate	0.00%
Mortgage-backed securities with fair value	1,600,000,000			1,700,000,000
Short-term Securities [Member]
Securities sold under agreement to repurchase, fair value	$ 11,500,000
Weighted average rate of securities sold under agreements	0.90%
Minimum [Member]
Long-term debt percentage bearing fixed interest rate	4.29%
Initial fixed rate	1.00%
Initial fixed rate period (in months)	6
Maximum interest rate	3.25%
Maximum [Member]
Long-term debt percentage bearing fixed interest rate	5.07%
Initial fixed rate	3.50%
Initial fixed rate period (in months)	18
Maximum interest rate	3.75%