Loans - Summary of Impaired Financial Receivables Excluding Purchased Non-Covered and Covered Loans (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings not included above	$ 1,519,000	$ 2,241,000	$ 1,212,000
Excluding Purchased Non-Covered and Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	20,841,000	26,729,000	21,728,000
Troubled debt restructurings not included above	12,935,000	18,848,000	12,759,000
Total impaired loans	33,776,000	45,577,000	34,487,000
Interest income recognized on impaired loans	168,000	246,000	1,991,000
Foregone interest income on impaired loans	$ 109,000	$ 246,000	$ 155,000