Borrowings - Schedule of Borrowings (Detail) - USD ($)	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Debt Disclosure [Abstract]
Securities sold under agreements with customers to repurchase	$ 339,799,000	$ 310,330,000	$ 346,369,000
Federal funds purchased	7,125,000	6,325,000	9,750,000
Advances from Federal Home Loan Bank of Dallas	210,000,000	299,020,000	265,036,000
Total	$ 556,924,000	$ 615,675,000	$ 621,155,000