Goodwill and Intangible Assets - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Goodwill And Core Deposit Intangibles [Line Items]
Core deposit intangible assets	$ 4,542,000	$ 4,383,000	$ 1,149,000
Aggregate amortization expense for intangible assets	2,330,000	1,414,000	1,359,000
Minimum [Member]
Goodwill And Core Deposit Intangibles [Line Items]
Amortization period of core deposit intangibles	3 years
Maximum [Member]
Goodwill And Core Deposit Intangibles [Line Items]
Amortization period of core deposit intangibles	10 years
Coastal Bankshares, Inc. [Member]
Goodwill And Core Deposit Intangibles [Line Items]
Goodwill	27,437,000
Prosperity Bank [Member]
Goodwill And Core Deposit Intangibles [Line Items]
Goodwill		$ 35,153,000