Loans - Summary of Impaired Financial Receivables (Summary of Covered Impaired Loans) (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 200,000
Troubled debt restructurings not included above	23,300,000	31,700,000	23,900,000
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	98,515,000	170,221,000	115,712,000
Troubled debt restructurings not included above	21,592,000	18,220,000	19,194,000
Total impaired loans	120,107,000	188,441,000	134,906,000
Impaired loans not requiring a related allowance	120,107,000	188,441,000	134,906,000
Impaired loans requiring a related allowance
Allowance related to impaired loans
Average investment in impaired loans	127,507,000	184,162,000	163,825,000
Interest income recognized on impaired loans	169,000	179,000	849,000
Foregone interest income on impaired loans	$ 147,000	$ 441,000	$ 491,000