Business Segments - Financial Information by Business Segment (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Segment Reporting Information [Line Items]    
Interest and dividend income $ 86,417 $ 79,924
Interest expense 13,125 9,205
Net interest income 73,292 70,719
Provision for credit losses 2,168 2,711
Net interest income after provision for credit losses 71,124 68,008
Non-interest income 23,980 22,884
Non-interest expense 54,571 54,384
Income before provision for income taxes 40,533 36,508
Provision for income taxes 7,004 10,622
Net income 33,529 25,886
Community Banking [Member]    
Segment Reporting Information [Line Items]    
Interest and dividend income 86,417 79,924
Interest expense 13,125 9,205
Net interest income 73,292 70,719
Provision for credit losses 2,168 2,711
Net interest income after provision for credit losses 71,124 68,008
Non-interest income 17,477 16,741
Non-interest expense 50,894 50,992
Income before provision for income taxes 37,707 33,757
Provision for income taxes 6,411 9,522
Net income 31,296 24,235
Trust and Investment Services [Member]    
Segment Reporting Information [Line Items]    
Non-interest income 6,503 6,143
Non-interest expense 3,677 3,392
Income before provision for income taxes 2,826 2,751
Provision for income taxes 593 1,100
Net income $ 2,233 $ 1,651