Annual report pursuant to Section 13 and 15(d)

Income Taxes - Additional Information (Detail)

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Income Taxes - Additional Information (Detail) - USD ($)
 12 Months Ended
Jan. 01, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Operating Loss Carryforwards [Line Items]        
Unrecognized tax benefits that would affect our effective tax rate if recognized   $ 73,000 $ 202,000 $ 1,000,000.0
Decrease in unrecognized tax benefits due to state taxes   129,000    
Decrease in unrecognized tax benefits due to lapse of statute of limitations   0 837,000 0
Accrued interest on uncertain tax benefits   0 10,000 34,000
Accrued interest related to uncertain tax positions   0 57,000 132,000
Valuation allowance   4,852,000 $ 4,852,000 $ 2,750,000
Federal and state income tax   $ 216,400,000    
Federal statutory income tax rate   21.00% 21.00% 35.00%
Change to provisional amount related to remeasurement of deferred tax assets and liabilities     $ 1,100,000  
Federal [Member]        
Operating Loss Carryforwards [Line Items]        
Net operating loss carryforwards   $ 17,300,000    
Federal [Member] | California [Member]        
Operating Loss Carryforwards [Line Items]        
Net operating loss carryforwards   152,300    
Federal [Member] | Illinois [Member]        
Operating Loss Carryforwards [Line Items]        
Net operating loss carryforwards   63,700    
State [Member]        
Operating Loss Carryforwards [Line Items]        
Low income housing tax credit   251,000    
Central Bancorp, Inc        
Operating Loss Carryforwards [Line Items]        
Increase (decrease) in deferred tax assets   $ 9,300,000 $ (5,000,000.0)  
ASU 2018-02        
Operating Loss Carryforwards [Line Items]        
Impact on retained earnings of adopting recent accounting pronouncement $ 399,000