Business Combinations - Estimated Fair Value of Assets Acquired and Liabilities Assumed (Parenthetical) (Detail) (Prosperity Banking Company [Member], USD $) In Thousands, unless otherwise specified	0 Months Ended
	Dec. 23, 2014	Dec. 23, 2014
As Recorded by Acquired Bank [Member]
Business Acquisition [Line Items]
Deferred tax asset, purchase accounting adjustments	$ 8,435	$ 8,435
Fair Value Adjustments [Member]
Business Acquisition [Line Items]
Deferred tax asset, non-realizable portion	6,644
Fair value adjustment of other assets	(599)	(599)
Fair value adjustment (premium) to FHLB borrowings	12,741	12,741
Fair value adjustment to subordinated debt	$ 428	$ 428