Business Combinations - Additional Information (Detail)				6 Months Ended
	Jun. 12, 2015 USD ($) Branches	May. 22, 2015 USD ($) Location	Jun. 30, 2014 USD ($) Location shares	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Goodwill, excess of the merger consideration over fair value of net assets			$ 58,903,000	$ 58,903,000	$ 87,367,000	$ 63,547,000
Issuance of common stock, fair value				34,474,000
Coastal Bank [Member]
Business Acquisition [Line Items]
Number of banking locations | Location			6
Coastal Bankshares, Inc. [Member]
Business Acquisition [Line Items]
Net of loans acquired			$ 16,700,000	16,700,000
Net of loans acquired			11,882,000	11,882,000
Goodwill, excess of the merger consideration over fair value of net assets			28,093,000	28,093,000	28,100,000
Fair value of loans acquired			$ 279,400,000	$ 279,400,000
Estimated discount			5.64%	5.64%
Impaired loans			$ 29,300,000	$ 29,300,000	3,282,000
Shares to receive for each share of Prosperity common stock pursuant to the merger agreement			0.4671	0.4671
Issuance of common stock, shares | shares			1,598,998
Issuance of common stock, fair value			$ 34,500,000
Coastal Bankshares, Inc. [Member] | As Recorded by Acquired Bank [Member]
Business Acquisition [Line Items]
Cash paid as deposit premium			2,796,000
Net of loans acquired			11,882,000	$ 11,882,000
Purchase price			$ 37,270,000
Issuance of common stock, shares | shares			1,598,998
Coastal Bankshares, Inc. [Member] | Warrant [Member]
Business Acquisition [Line Items]
Cash in exchange for outstanding warrants			$ 2,800,000	$ 2,800,000
Merchants and Southern Banks of Florida [Member]
Business Acquisition [Line Items]
Net of loans acquired		$ 8,500,000
Net of loans acquired		10,934,000
Goodwill, excess of the merger consideration over fair value of net assets		12,090,000			12,100,000
Fair value of loans acquired		191,500,000
Purchase price		$ 50,000,000
Number of banking locations | Location		13
Estimated discount		4.25%
Impaired loans		$ 17,400,000			3,254,000
Merchants and Southern Banks of Florida [Member] | As Recorded by Acquired Bank [Member]
Business Acquisition [Line Items]
Cash paid as deposit premium		50,000,000
Net of loans acquired		14,614,000
Purchase price		$ 50,000,000
Bank of America [Member]
Business Acquisition [Line Items]
Number of branches | Branches	18
Deposit premium	3.00%
Net of loans acquired	$ 4,400,000
Cash paid as deposit premium	$ 20,000,000
Deposits period	15 days
Net of loans acquired	$ 11,408,000
Goodwill, excess of the merger consideration over fair value of net assets	11,076,000				$ 11,100,000
Fair value of loans acquired	4,400,000
Bank of America [Member] | As Recorded by Acquired Bank [Member]
Business Acquisition [Line Items]
Cash paid as deposit premium	20,002,000
Net of loans acquired	10,348,000
Purchase price	$ 20,002,000