Acquisitions Definitive Agreement Signed - Additional Information (Detail) - USD ($)	Oct. 12, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Business Acquisition [Line Items]
Gross loans		$ 3,472,227,000	$ 3,357,307,000	$ 3,337,793,000
Total deposits		5,697,460,000	5,478,539,000	5,235,464,000
Total assets		7,009,164,000	$ 6,809,931,000	$ 6,686,736,000
Commercial State Bank [Member]
Business Acquisition [Line Items]
Gross loans		263,800,000
Total deposits		322,100,000
Total assets		$ 366,800,000
Subsequent Event [Member] | Commercial Bancshares, Inc [Member]
Business Acquisition [Line Items]
Business acquisition, agreement date	Oct. 12, 2017
Business acquisition, expected stock purchase price	$ 59,400,000
Business acquisition, special dividend	15,600,000
Increase decrease in business acquisition, special dividend	$ 42,400,000