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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Significant Accounting Policies [Line Items]        
Cash and cash equivalents, interest-bearing deposits in other financial institutions $ 557,850,000 $ 537,674,000 $ 564,150,000 $ 644,550,000
Cash and cash equivalents, interest-bearing deposits in other financial institutions, Federal Reserve Bank of New York 138,600,000 119,200,000    
Cash and cash equivalents, federal funds sold 6,800,000 4,600,000    
Cash and cash equivalents, outstanding reverse repurchase agreements 250,000,000 250,000,000    
Minimum amount required to be maintained as total reserves with the Federal Reserve Bank of New York 162,100,000 158,300,000    
Goodwill impairment losses 0 0 0  
Depreciation and amortization 32,811,000 31,497,000 27,792,000  
Bank-owned life insurance 949,026,000 931,627,000    
Bank-owned life insurance income 31,015,000 27,541,000 $ 27,150,000  
Purchase of bank-owned life insurance policy 0 0    
Other real estate owned 28,598,000 39,882,000    
Other real estate owned, covered by FDIC loss sharing agreements $ 16,990,000 $ 25,817,000    
Shares available for grant 9,646,760      
Building        
Significant Accounting Policies [Line Items]        
Premises, furniture, fixtures, and equipment, estimated useful lives (in years) 20 years      
Options and Restricted Stock        
Significant Accounting Policies [Line Items]        
Shares available for grant 9,646,760      
Two Thousand Twelve Incentive Plan        
Significant Accounting Policies [Line Items]        
Shares transferred 1,030,673      
AmTrust Bank        
Significant Accounting Policies [Line Items]        
Loss sharing agreements, expiration 2019-12      
Desert Hills Bank        
Significant Accounting Policies [Line Items]        
Loss sharing agreements, expiration 2020-03      
Minimum | Furniture and Fixtures        
Significant Accounting Policies [Line Items]        
Premises, furniture, fixtures, and equipment, estimated useful lives (in years) 3 years      
Minimum | Equipment        
Significant Accounting Policies [Line Items]        
Premises, furniture, fixtures, and equipment, estimated useful lives (in years) 3 years      
Maximum | Furniture and Fixtures        
Significant Accounting Policies [Line Items]        
Premises, furniture, fixtures, and equipment, estimated useful lives (in years) 10 years      
Maximum | Equipment        
Significant Accounting Policies [Line Items]        
Premises, furniture, fixtures, and equipment, estimated useful lives (in years) 10 years      
Core deposit intangibles | Maximum        
Significant Accounting Policies [Line Items]        
Core deposit intangible, estimated useful lives (in years) 10 years