Loans and leases and the allowance for credit losses (Details Textual) (USD $)	3 Months Ended	6 Months Ended	12 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Mar. 31, 2012	Jun. 30, 2012 Acquired Loans [Member]	Jun. 30, 2011 Acquired Loans [Member]	Jun. 30, 2012 Acquired Loans [Member]	Jun. 30, 2011 Acquired Loans [Member]	Dec. 31, 2010 Acquired Loans [Member]
Loans and Leases Receivable [Line Items]
Interest income	$ 89,870,000	$ 171,257,000			$ 90,000,000	$ 69,000,000	$ 171,000,000	$ 110,000,000
Accretable yield balance	788,760,000	788,760,000	838,765,000	770,031,000		1,040,000,000		1,040,000,000	457,000,000
Percentage of improvement in acquired loans							1.00%
Loans and leases and allowance for credit losses (Details) (Textual) [Abstract]
Purchased impaired loans as a percentage of total assets		Less than 1% of company's assets	Less than 1% of company's assets
Contractual principal and interest payments	561,000,000	561,000,000	653,000,000
Transfer from nonaccretable to accretable yield		$ 140,000,000
Loan delinquent period		90 days
Maturity period of line of credit		more than 150 days