Short-Term Borrowings - Schedule of Short-term Borrowings (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Debt Disclosure [Abstract]
Securities sold under agreements with customers to repurchase	$ 324,150,000	$ 357,400,000	$ 330,846,000
Federal funds purchased	6,725,000	8,700,000	10,025,000
Advances from Federal Home Loan Bank of Dallas	170,028,000	1,010,000	1,038,000
Total	$ 500,903,000	$ 367,110,000	$ 341,909,000