Borrowings - Schedule of Borrowings (Detail) - USD ($)	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Debt Disclosure [Abstract]
Securities sold under agreements with customers to repurchase	$ 339,660,000	$ 360,820,000	$ 345,559,000
Federal funds purchased	11,775,000	9,950,000	8,200,000
Advances from Federal Home Loan Bank of Dallas	0	75,000,000	160,000,000
Total	$ 351,435,000	$ 445,770,000	$ 513,759,000