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Business Combinations - Additional Information (Detail)
3 Months Ended 6 Months Ended
Aug. 10, 2018
USD ($)
Branch
Jun. 30, 2019
USD ($)
Branch
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
Branch
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Business Acquisition [Line Items]            
Goodwill   $ 663,707,000   $ 663,707,000   $ 666,539,000
Merger related expenses   $ 2,612,000 $ 494,000 $ 5,761,000 $ 1,297,000  
Community Bank [Member]            
Business Acquisition [Line Items]            
Acquisition date       Aug. 10, 2018    
Number of branches owned by merger company | Branch 16 10   10    
Acquired assets and assumed liabilities for cash $ 180,719,000          
Assets and assumed all of the liabilities, stock 722,767,000          
Goodwill 547,144,000          
Core deposit intangible assets $ 52,200,000          
Percentage of core deposits to total deposits 2.26%          
Merger related expenses   $ 2,600,000 $ 494,000 $ 5,800,000 $ 1,300,000  
Gross contractual amount receivable, as of acquisition date $ 3,000,000,000          
Contractual principal cash flows, loans not expected to be collected $ 4,500,000