Business Combinations - Additional Information (Detail) (USD $)	9 Months Ended		12 Months Ended	0 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended	0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Dec. 23, 2013 Prosperity Bank [Member] Location	Dec. 23, 2013 Prosperity Banking Company [Member]	Sep. 30, 2014 Prosperity Banking Company [Member]	Sep. 30, 2013 Prosperity Banking Company [Member]	Dec. 31, 2013 Prosperity Banking Company [Member]	Jun. 30, 2014 Coastal Bank [Member] Location	Jun. 30, 2014 Coastal Bankshares, Inc. [Member]	Sep. 30, 2014 Coastal Bankshares, Inc. [Member]	Jun. 30, 2014 Coastal Bankshares, Inc. [Member]	Jun. 30, 2014 Coastal Bankshares, Inc. [Member] Warrant [Member]
Business Acquisition [Line Items]
Number of banking locations				12					6
Shares to receive for each share of Prosperity common stock pursuant to the merger agreement					3.125							0.4671
Issuance of common stock, shares					1,168,918					1,598,998
Issuance of common stock, fair value					$ 24,600,000					$ 34,500,000
Cash in exchange for outstanding warrants													2,800,000
Good will, excess of the merger consideration over fair value of net assets	58,879,000	956,000	35,049,000		34,093,000	34,100,000						23,830,000
Fair value of loans acquired	449,700,000											279,400,000
Net of loans acquired	37,700,000											16,700,000
Estimated discount	7.73%											5.64%
Impaired loans	67,200,000					(10,985,000)					(3,282,000)	29,300,000
Cash to receive for each share of Prosperity common stock pursuant to the merger agreement					$ 41.50
Provision for loan loss expense	$ 685,000	$ 1,300,000	$ 1,500,000			$ 4,000,000	$ 0	$ 0