The aggregate excess of a loan's cash flows expected to be collected over the investor's initial investment in the loan. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

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The price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

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Loans and leases and the allowance for credit losses - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended	12 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Dec. 31, 2011	Jun. 30, 2012	Sep. 30, 2012 Acquired Loans [Member]	Sep. 30, 2011 Acquired Loans [Member]	Sep. 30, 2012 Acquired Loans [Member]	Sep. 30, 2011 Acquired Loans [Member]	Dec. 31, 2010 Acquired Loans [Member]
Loans And Leases Receivable [Line Items]
Contractual principal and interest payments	$ 528,000,000	$ 528,000,000	$ 653,000,000
Purchased impaired loans as a percentage of total assets		Less than 1% of the Company's assets	Less than 1% of the Company's assets
Interest income	87,372,000	258,629,000			87,000,000	97,000,000	259,000,000	207,000,000
Accretable yield balance	$ 703,312,000	$ 703,312,000	$ 838,765,000	$ 788,760,000		$ 944,000,000		$ 944,000,000	$ 457,000,000
Loan delinquent period		90 days
Maturity period of line of credit		More than 150 days