Loans - Summary of Impaired Financial Receivables (Summary of Non-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
Non-Covered Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	37,476,000	52,258,000	38,885,000
Troubled debt restructurings not included above	18,513,000	26,848,000	18,744,000
Total impaired loans	55,989,000	79,106,000	57,629,000
Impaired loans not requiring a related allowance
Impaired loans requiring a related allowance	55,989,000	79,106,000	57,629,000
Allowance related to impaired loans	4,839,000	9,500,000	5,115,000
Average investment in impaired loans	56,808,000	83,940,000	70,209,000
Interest income recognized on impaired loans	78,000	57,000	495,000
Foregone interest income on impaired loans	$ 54,000	$ 187,000	$ 718,000