Business Combinations - Summary of Contractually Required Principal and Interest Cash Payment of the Loans As of Acquisition Date for Purchased Credit Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2012	Jun. 30, 2014
Business Acquisition [Line Items]
Contractually required principal and interest		$ 137,407
Non-accretable difference		(53,603)
Cash flows expected to be collected		83,804
Fair value of loans acquired with deterioration of credit quality	67,165	73,414
Coastal Bankshares, Inc. [Member]
Business Acquisition [Line Items]
Contractually required principal and interest	38,194
Non-accretable difference	(5,632)
Cash flows expected to be collected	32,562
Accretable yield	(3,282)		(29,300)
Fair value of loans acquired with deterioration of credit quality	29,280
Prosperity Banking Company [Member]
Business Acquisition [Line Items]
Contractually required principal and interest	92,461
Non-accretable difference	(14,311)
Cash flows expected to be collected	78,150
Accretable yield	(10,985)
Fair value of loans acquired with deterioration of credit quality	$ 67,165