Quarterly report pursuant to Section 13 or 15(d)

Securities - Additional Information (Detail)

v3.10.0.1
Securities - Additional Information (Detail) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Jan. 01, 2018
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]            
Impairment charges on securities in continuous unrealized loss position for 12 months or longer       $ 0   $ 0
Reduction in securities available for sale   $ (565,529,000)   (565,529,000)   (578,804,000)
Reduction to retained earnings   (85,465,000)   (85,465,000)   (70,575,000)
Increase in net deferred tax asset   35,500,000   35,500,000   32,500,000
Gains included in earnings       67,000    
Gross realized losses included in earnings   428,000   957,000    
Net unrealized gains (losses) recorded in comprehensive income   95,000 $ 430,000 95,000 $ 744,000  
Net gain in earnings resulting from sale of securities         $ 1,200,000  
Net gain in earnings resulting from sale of securities   67,000 $ 938,000      
Available for sale securities pledged as collateral   $ 18,100,000   $ 18,100,000   $ 130,100,000
ASU 2016-01 [Member]            
Debt Securities, Available-for-sale [Line Items]            
Reduction in securities available for sale $ 529,000          
Reduction to retained earnings 382,000          
Increase in net deferred tax asset 147,000          
Net unrealized gains (losses) recorded in comprehensive income $ (529,000)