Loans - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Outstanding loans to Executive officers, directors, principal shareholders, related interest and parties	$ 93,700,000		$ 93,700,000		$ 105,600,000
Non-Covered Loans	36,778,401,000		36,778,401,000		35,740,489,000
Covered loans	1,890,883,000		1,890,883,000		2,060,089,000
Provision for (recovery of) losses	895,000	$ 334,000	719,000	$ 341,000
FDIC indemnification expense	1,500,000		3,800,000
FDIC indemnification income		1,800,000		2,500,000
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			15,600,000
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			2,900,000
Purchased Credit-Impaired Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Covered Loans	5,983,000		5,983,000		8,344,000
Purchased credit-impaired loans outstanding	7,500,000		7,500,000
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	1,700,000,000		1,700,000,000
Provision for (recovery of) losses	(1,849,000)	$ 2,206,000	(4,746,000)	$ 3,083,000
Covered Loans | Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	133,139,000		133,139,000		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	27,700,000		27,700,000
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	$ 2,300,000,000		$ 2,300,000,000		$ 2,500,000,000