Borrowings - Schedule of Borrowings (Detail) - USD ($)	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Debt Disclosure [Abstract]
Securities sold under agreements with customers to repurchase	$ 342,540,000	$ 310,330,000	$ 366,140,000
Federal funds purchased	7,800,000	6,325,000	19,775,000
Advances from Federal Home Loan Bank of Dallas	175,000,000	299,020,000	15,983,000
Total	$ 525,340,000	$ 615,675,000	$ 401,898,000