Loans - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Outstanding loans to Executive officers, directors, principal shareholders, related interest and parties	$ 92,300,000		$ 92,300,000		$ 105,600,000
Non-Covered Loans	37,335,541,000		37,335,541,000		35,740,489,000
Covered loans	1,789,164,000		1,789,164,000		2,060,089,000
Provision for (recovery of) losses	(55,000)	$ (9,028,000)	664,000	$ (8,687,000)
FDIC indemnification expense	1,000,000	6,800,000	4,800,000	4,300,000
Financing Receivable Troubled Debt Restructurings Rate Reductions
Financing Receivable, Recorded Investment [Line Items]
Delinquent loans selectively extended to certain borrowers, rate reductions, forbearance of arrears, and extension of maturity dates			16,600,000
Financing Receivable Troubled Debt Restructurings Forbearance of Arrears
Financing Receivable, Recorded Investment [Line Items]
Delinquent loans selectively extended to certain borrowers, rate reductions, forbearance of arrears, and extension of maturity dates			2,800,000
Purchased Credit-Impaired Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Covered Loans	5,939,000		5,939,000		8,344,000
Purchased credit-impaired loans outstanding	7,300,000		7,300,000
Principal shareholders
Financing Receivable, Recorded Investment [Line Items]
Outstanding loans to Executive officers, directors, principal shareholders, related interest and parties	0		0		0
Covered Loans
Financing Receivable, Recorded Investment [Line Items]
Current	1,600,000,000		1,600,000,000
Provision for (recovery of) losses	(1,289,000)	$ (8,516,000)	(6,035,000)	$ (5,433,000)
Covered Loans | Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	133,077,000		133,077,000		137,182,000
Covered Loans | Performing Financial Instruments | Financing Receivable Recorded Investment 30 to 89 days past due
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	26,400,000		26,400,000
Covered Loans | Performing Financial Instruments | Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	133,100,000		133,100,000
Am Trust Bank and Desert Hills Bank
Financing Receivable, Recorded Investment [Line Items]
Total loans, net	$ 2,200,000,000		$ 2,200,000,000		$ 2,500,000,000