Business Combinations - Summary of Contractually Required Principal and Interest Cash Paymentof the Loans As of Acquisition Date for Purchased Credit Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Jun. 30, 2014
Loans At Acquisition Date [Line Items]
Accretable yield	$ 67,200
Coastal Bankshares, Inc. [Member]
Loans At Acquisition Date [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
Total purchased credit-impaired loans acquired	29,280
Prosperity Banking Company [Member]
Loans At Acquisition Date [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)
Total purchased credit-impaired loans acquired	$ 67,165