Loans and Allowance for Credit Losses - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Percentage covered up to $467.2 million	0.00%
Loan placed on nonaccrual when past due, number of days	90 days
Number of months required for loan to return to accrual status	6 months
Loan participations	$ 309,300,000
Loan participations sold	170,500,000
Loan participations retained	138,800,000
Troubled debt restructuring term	6 months
Minimum number of days for loan charge off to be recorded	120 days
Maximum number of days for loan charge off to be recorded	180 days
Nonaccrual period for loans	90 days
Troubled debt restructurings	35,598,000	46,575,000	34,953,000	48,867,000
Financing receivable TDR's included with non-accrual loans	23,100,000		22,100,000
Financing receivable troubled debt restructurings specific reserves	1,600,000		2,800,000
Unfunded commitments on TDRs	1,600,000
Number of days for a loan to be considered to be in payment default	90 days
Troubled debt restructurings which was a payment default	300,000	1,400,000
Troubled debt restructuring modification payment delay	90 days
Outstanding noncovered loans including principal, interest, fees and penalties	132,700,000		135,900,000
Accretion recorded as loan interest income	2,900,000	6,400,000
Financing receivable allowance for loan losses related to purchased loans	1,700,000		1,500,000
Maximum [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Value of small commercial loans on nonaccrual status or 90 days or more delinquent	250,000
Credit Concentration Risk [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Concentration of commercial loans in a single industry, maximum	10.00%
Losses up to $275.0 [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loss amount covered at 80% up to $467.2 million	80.00%
Loss amount covered at 80% up to $275 million	275,000,000
Losses in Excess of $467.2 [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loss amount covered at 80% up to $467.2 million	80.00%
Loss amount covered at 80% up to $275 million	467,200,000
Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Troubled debt restructurings	15,334,000	21,621,000	15,205,000	22,443,000
Number of loans modified as TDRs	3	4
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Troubled debt restructurings	15,439,000	20,959,000	15,226,000	22,639,000
Number of loans modified as TDRs	1	2
Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Troubled debt restructurings	2,458,000	1,656,000	2,459,000	1,441,000
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Troubled debt restructurings	$ 2,367,000	$ 2,339,000	$ 2,063,000	$ 2,344,000