Business Combinations - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) Branch	Feb. 08, 2019 USD ($) Branch	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Goodwill	$ 136,432		$ 131,712
MidFirst Bank [Member]
Business Acquisition [Line Items]
Acquisition-related costs	626
Acquisition-related costs after tax	477
Goodwill	$ 4,720	$ 4,720
Number of branches acquired | Branch	3
MidFirst Bank [Member] | Guymon, Oklahoma [Member]
Business Acquisition [Line Items]
Number of branches | Branch		2
MidFirst Bank [Member] | Cordell, Oklahoma [Member]
Business Acquisition [Line Items]
Number of branches | Branch		1