Loans and Allowance for Credit Losses - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Concentration of commercial loans in a single industry, maximum			10.00%
Accrued interest coverage by FDIC, number of days			90 days
Percentage covered up to $467.2 million	0.00%		0.00%
Number of years for loss sharing provisions for commercial mortgage loans			5 years
Number of years for loss sharing provisions for single family residential mortgage loans			10 years
Number of years for loss recovery provisions for commercial mortgage loans			8 years
Number of years for loss recovery provisions for single family residential mortgage loans			10 years
Average balance of impaired loans	$ 71,028,000	$ 66,309,000	$ 71,878,000	$ 76,677,000
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	223,800,000		223,800,000
Loan participations sold	142,900,000		142,900,000
Loan participations retained	80,900,000		80,900,000
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
Troubled debt restructurings			46,500,000		32,100,000
Financing receivable TDR's included with non-accrual loans			31,000,000		22,100,000
Financing receivable troubled debt restructurings specific reserves			5,400,000		4,500,000
Charge-offs			0		1,000,000
Increased allowance of loan losses			2,500,000		400,000
Number of days for a loan to be considered to be in payment default			90 days
Troubled debt restructuring modification payment delay			90 days
Carrying value of revolving lines of credit loans not impaired	4,000,000		4,000,000
Outstanding noncovered loans including principal, interest, fees and penalties	129,500,000		129,500,000		179,500,000
Accretion recorded as loan interest income			11,900,000	7,900,000
Financing receivable allowance for loan losses related to purchased loans			3,100,000		4,300,000
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
Losses up to $275.0 [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loss sharing reimbursement threshold			80.00%
Loss amount covered at 80% up to $275 million	275,000,000		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 $275 million	467,200,000		467,200,000
Loss amount covered at 80% up to $467.2 million			80.00%
Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Troubled debt restructurings			21,400,000		12,700,000
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Troubled debt restructurings			22,700,000		18,400,000
Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Troubled debt restructurings			900,000		500,000
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Troubled debt restructurings			$ 1,500,000		$ 500,000