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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Sep. 30, 2013
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2013
Summary Of Significant Accounting Policies [Line Items]        
Maturity period of highly liquid debt instruments   Three months or less    
Available-for-sale securities transferred to held-to-maturity securities $ 536,000,000     $ 536,000,000
Unrealized net loss on securities reclassified from available-for-sale to held-to-maturity, Gross $ 13,300,000     $ 13,300,000
Loans past due, minimum period   90 days    
Non-accrual business and commercial real estate loans   $ 250,000    
Delinquency period of losses on mortgage loan to be deemed uncollectible   120 days    
Allowance for loan losses   $ 0    
Mortgage loans sold on real estate, final maturity period   15 years    
Delinquency period of mortgage loans   90 days    
Share-based compensation, excess tax benefit, amount     $ 1,800,000  
Computer Equipment        
Summary Of Significant Accounting Policies [Line Items]        
Estimated depreciable life of assets   3 years    
Furniture, Fixtures and Other Electronic Equipment        
Summary Of Significant Accounting Policies [Line Items]        
Estimated depreciable life of assets   5 years    
Building Improvements        
Summary Of Significant Accounting Policies [Line Items]        
Estimated depreciable life of assets   10 years    
Buildings        
Summary Of Significant Accounting Policies [Line Items]        
Estimated depreciable life of assets   30 years    
Retained Earnings | Accounting Standards Update 2014-09        
Summary Of Significant Accounting Policies [Line Items]        
Equity reclassification     $ 147,000  
Minimum | Accounting Standards Update 2016-02        
Summary Of Significant Accounting Policies [Line Items]        
Operating lease, liability   $ 18,000,000    
Operating lease, right-of-use asset   18,000,000    
Maximum | Accounting Standards Update 2016-02        
Summary Of Significant Accounting Policies [Line Items]        
Operating lease, liability   22,000,000    
Operating lease, right-of-use asset   $ 22,000,000