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Condensed Financial Information of Parent Company - Condensed Statements of Earnings (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]                      
Other expense, net                 $ (140,753) $ (136,740) $ (140,659)
Net earnings $ 17,851 $ 29,683 $ 28,373 $ 28,504 $ 27,076 $ 25,448 $ 25,514 $ 23,391 104,411 101,429 99,145
Parent Company [Member]                      
Condensed Financial Statements, Captions [Line Items]                      
Excess in net earnings of subsidiaries                 50,253 55,192 53,259
Dividends from the Bank                 57,000 49,000 49,000
Other expense, net                 (2,842) (2,763) (3,114)
Net earnings                 $ 104,411 $ 101,429 $ 99,145