Borrowings - Schedule of Borrowings (Detail) - USD ($)	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Debt Disclosure [Abstract]
Securities sold under agreements with customers to repurchase	$ 347,339,000	$ 360,820,000	$ 342,540,000
Federal funds purchased	12,925,000	9,950,000	7,800,000
Advances from Federal Home Loan Bank of Dallas	0	75,000,000	175,000,000
Total	$ 360,264,000	$ 445,770,000	$ 525,340,000