Borrowings - Schedule of Borrowings (Detail) - USD ($)	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Debt Disclosure [Abstract]
Securities sold under agreements with customers to repurchase	$ 345,559,000	$ 310,330,000	$ 324,150,000
Federal funds purchased	8,200,000	6,325,000	6,725,000
Advances from Federal Home Loan Bank of Dallas	160,000,000	299,020,000	170,028,000
Total	$ 513,759,000	$ 615,675,000	$ 500,903,000