Business Combinations - Pro Forma Information of Acquisitions (Detail) - Coastal Bankshares, Inc. [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Business Acquisition [Line Items]
Net interest income and noninterest income	$ 63,259	$ 60,212	$ 123,308	$ 116,454
Net income (loss)	(128)	6,963	10,739	17,163
Net income (loss) available to common stockholders	$ (128)	$ 6,963	$ 10,739	$ 16,877
Income (loss) per common share available to common stockholders - basic	$ 0.00	$ 0.26	$ 0.34	$ 0.63
Income (loss) per common share available to common stockholders - diluted	$ 0.00	$ 0.26	$ 0.34	$ 0.62
Average number of shares outstanding, basic	32,184	26,780	31,318	26,762
Average number of shares outstanding, diluted	32,520	27,232	31,653	27,214