Business Combinations - Pro Forma Information of Acquisitions (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2014 Coastal Bankshares, Inc. [Member]	Sep. 30, 2014 Coastal Bankshares, Inc. [Member]	Sep. 30, 2013 Coastal Bankshares, Inc. [Member]	Sep. 30, 2013 Prosperity Banking Company [Member]	Sep. 30, 2013 Prosperity Banking Company [Member]
Loans At Acquisition Date [Line Items]
Net interest income and noninterest income	$ 46,373	$ 165,913	$ 137,590	$ 48,541	$ 142,390
Net income	6,680	26,275	19,733	7,214	18,729
Net income available to common stockholders	$ 6,237	$ 25,989	$ 18,407	$ 6,771	$ 17,403
Income per common share available to common stockholders - basic	$ 0.24	$ 0.95	$ 0.72	$ 0.27	$ 0.69
Income per common share available to common stockholders - diluted	$ 0.24	$ 0.94	$ 0.71	$ 0.27	$ 0.68
Average number of shares outstanding, basic	25,500	27,304	25,482	25,070	25,052
Average number of shares outstanding, diluted	25,915	27,698	25,897	25,485	25,467