Borrowings (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Amounts and interest rate of short-term borrowings
Federal funds purchased and short term borrowings	$ 782,082	$ 947,432	$ 2,442,582
Federal funds purchased and repurchase agreements	732,059	866,555	2,211,692
Other short-term borrowings	50,023	80,877	230,890
Weighted-average interest rate	0.09%	0.19%	0.10%
Daily-average amount outstanding	826,808	1,854,033	2,911,065
Weighted-average interest rate	0.12%	0.16%	0.24%
Federal Funds Purchased and Repurchase Agreements [Member]
Amounts and interest rate of short-term borrowings
Weighted-average interest rate	0.04%	0.19%	0.04%
Highest amount at a month-end	1,389,237	2,612,727	2,491,573
Daily-average amount outstanding	706,749	1,749,525	1,885,464
Weighted-average interest rate	0.11%	0.15%	0.15%
Other Short Term Borrowings [Member]
Amounts and interest rate of short-term borrowings
Weighted-average interest rate	0.70%	0.20%	0.66%
Highest amount at a month-end	142,927	236,842	2,049,727
Daily-average amount outstanding	$ 120,059	$ 104,508	$ 1,025,601
Weighted-average interest rate	0.18%	0.33%	0.42%