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Summary of Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2018
USD ($)
Segment
Office
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Oct. 16, 2009
Summary Of Significant Accounting Policies [Line Items]        
Number of operating business segments | Segment 2      
Net unrealized holding gain after tax $ (20,092,000) $ (8,719,000) $ (10,738,000)  
Number of days on which collection of interest on principal is no longer probable 90 days      
Period over which borrower has to demonstrate repayment performance in compliance with restructured terms to take loan out of nonaccrual status 6 months      
Original terms of modified loans 6 months      
Number of days on which charge-offs unsecured consumer loans are recorded 120 days      
Minimum loan amount for credit review process $ 1,000,000      
Allowance for loan losses look-back period, revised Rolling 20-quarters      
Agreed percentage of losses in loss sharing agreement       80.00%
Commercial loans expiration date Oct. 16, 2014      
Goodwill impairment $ 0      
Number of offices providing services, CitizensTrust | Office 3      
Assets under administration, CitizensTrust $ 2,540,000,000      
Assets under management, CitizensTrust $ 1,800,000,000      
Single-family Residential Loans [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Commercial loans expiration date Oct. 16, 2019      
Minimum [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Number of days over which principal or interest payments are past due to consider loans, excluding PCI loans 30 days      
Number of days for which principal or interest payments on loans, excluding PCI loans remain accrual 30 days      
Length of modified debt term 3 months      
Maximum [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Number of days for which principal or interest payments on loans, excluding PCI loans remain accrual 89 days