Business Combinations - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 23, 2013 Location	Jun. 30, 2014 Location
Business Acquisition [Line Items]
Issuance of common stock, fair value	$ 34,474,000	$ 24,629,000
Good will, excess of the merger consideration over fair value of net assets	63,547,000	35,049,000
Fair value of loans acquired	449,700,000
Net of loans acquired	37,700,000
Estimated discount	7.73%
Impaired loans	67,165,000		73,414,000
Provision for loan loss expense	843,000	1,500,000	2,600,000
Prosperity Bank [Member]
Business Acquisition [Line Items]
Number of banking locations				12
Prosperity Banking Company [Member]
Business Acquisition [Line Items]
Shares to receive for each share of Prosperity common stock pursuant to the merger agreement				3.125
Issuance of common stock, shares				1,168,918
Issuance of common stock, fair value				24,600,000
Good will, excess of the merger consideration over fair value of net assets	35,200,000			35,153,000
Impaired loans	10,985,000
Cash to receive for each share of Prosperity common stock pursuant to the merger agreement				41.50
Impaired loans	67,165,000
Provision for loan loss expense	84,000	0
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
Good will, excess of the merger consideration over fair value of net assets	27,400,000				27,437,000
Fair value of loans acquired					279,400,000
Net of loans acquired					16,700,000
Estimated discount					5.64%
Impaired loans	3,282,000				29,300,000
Impaired loans	29,280,000
Coastal Bankshares, Inc. [Member] | Warrant [Member]
Business Acquisition [Line Items]
Cash in exchange for outstanding warrants					2,800,000