Business Combinations - Additional Information (Detail) (USD $)	0 Months Ended
	Jan. 28, 2015 Location	Jun. 30, 2014 Location	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Business Acquisition [Line Items]
Number of banking locations	13
Goodwill, excess of the merger consideration over fair value of net assets			$ 63,547,000	$ 63,547,000	$ 35,049,000
Coastal Bank [Member]
Business Acquisition [Line Items]
Number of banking locations		6
Coastal Bankshares, Inc. [Member]
Business Acquisition [Line Items]
Shares to receive for each share of Prosperity common stock pursuant to the merger agreement		0.4671
Issuance of common stock, shares		1,598,998
Issuance of common stock, fair value		34,500,000
Goodwill, excess of the merger consideration over fair value of net assets		27,437,000	27,400,000
Fair value of loans acquired		279,400,000
Net of loans acquired		16,700,000
Estimated discount		5.64%
Impaired loans		29,300,000	3,282,000
Coastal Bankshares, Inc. [Member] | Warrant [Member]
Business Acquisition [Line Items]
Cash in exchange for outstanding warrants		2,800,000