Subordinated Deferrable Interest Debentures - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2005	Dec. 31, 2013	Dec. 31, 2006
Preferred Stock [Line Items]
Trust preferred securities interest rate	3.03%
Principal amount outstanding	$ 5,000,000
Aggregate principal amount	5,155,000
Principal amount issued			28,000,000
First National Banc Statutory Trust I [Member]
Preferred Stock [Line Items]
Principal amount of trust preferred securities		5,000,000
Variable rate of preferred securities		2.80%
Maturity period of trust preferred securities	30 years
Investment in common stock trust	155,000
Prosperity Banking Capital Trust I [Member]
Preferred Stock [Line Items]
Principal amount of trust preferred securities			5,000,000
Variable rate of preferred securities			2.57%
Trust preferred securities interest rate	2.80%
Maturity period of trust preferred securities	30 years
Principal amount outstanding	5,000,000
Aggregate principal amount	5,155,000
Investment in common stock trust	155,000
Fair value of debenture	3,158,000
Prosperity Banking Capital Trust II [Member]
Preferred Stock [Line Items]
Principal amount of trust preferred securities			4,500,000
Variable rate of preferred securities			3.15%
Trust preferred securities interest rate	3.40%
Maturity period of trust preferred securities	30 years
Principal amount outstanding	4,500,000
Aggregate principal amount	4,640,000
Investment in common stock trust	140,000
Fair value of debenture	3,196,000
Prosperity Bank Statutory Trust III [Member]
Preferred Stock [Line Items]
Principal amount of trust preferred securities			10,000,000
Variable rate of preferred securities			1.60%
Trust preferred securities interest rate	1.84%
Maturity period of trust preferred securities	30 years
Principal amount outstanding	10,000,000
Aggregate principal amount	10,310,000
Investment in common stock trust	310,000
Fair value of debenture	4,977,000
Prosperity Bank Statutory Trust IV [Member]
Preferred Stock [Line Items]
Principal amount of trust preferred securities			10,000,000
Variable rate of preferred securities			1.54%
Trust preferred securities interest rate	1.78%
Maturity period of trust preferred securities	30 years
Principal amount outstanding	10,000,000
Aggregate principal amount	10,310,000
Investment in common stock trust	310,000
Fair value of debenture	2,515,000
Coastal Bankshares Statutory Trust I [Member]
Preferred Stock [Line Items]
Principal amount of trust preferred securities	5,000,000
Variable rate of preferred securities	3.15%
Trust preferred securities interest rate	3.38%
Maturity period of trust preferred securities	30 years
Principal amount outstanding	5,000,000
Aggregate principal amount	10,310,000
Investment in common stock trust	155,000
Fair value of debenture	3,704,000
Coastal Bankshares Statutory Trust II [Member]
Preferred Stock [Line Items]
Principal amount of trust preferred securities	10,000,000
Variable rate of preferred securities	1.60%
Trust preferred securities interest rate	1.84%
Maturity period of trust preferred securities	30 years
Principal amount outstanding	10,000,000
Aggregate principal amount	10,310,000
Investment in common stock trust	310,000
Fair value of debenture	5,507,000
Ameris Statutory Trust I [Member]
Preferred Stock [Line Items]
Variable rate of preferred securities				1.63%
Aggregate principal amount				37,114,000
Investment in common stock trust	1,114,000
Principal amount issued				$ 36,000,000
Rate per annum of principal amount	1.87%
Trust preferred securities maturity date	Dec. 15, 2036