Business Combinations - Pro Forma Information of Acquisitions (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Net interest income and noninterest income	$ 176,262
Net income	(10,233)
Average number of shares outstanding, basic	23,816
Average number of shares outstanding, diluted	23,857
Coastal Bankshares, Inc. [Member]
Business Acquisition [Line Items]
Net interest income and noninterest income		223,281	183,459
Net income		36,855	21,397
Net income available to common shareholders		36,569	19,659
Income per common share available to common stockholders - basic		$ 1.33	$ 0.77
Income per common share available to common stockholders - diluted		$ 1.31	$ 0.76
Average number of shares outstanding, basic		27,573	25,517
Average number of shares outstanding, diluted		27,858	25,947
Prosperity Banking Company [Member]
Business Acquisition [Line Items]
Net interest income and noninterest income	199,089		187,927
Net income	15,604		19,927
Net income available to common shareholders	$ 12,027		$ 18,189
Income per common share available to common stockholders - basic	$ 0.48		$ 0.73
Income per common share available to common stockholders - diluted	$ 0.48		$ 0.71
Average number of shares outstanding, basic	24,985		25,087
Average number of shares outstanding, diluted	25,026		25,634