Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI (5) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	$ 15,146,000		$ 12,606,000
Past Due 60 - 89 Days	3,145,000		2,839,000
Past Due Greater Than 90 Days	2,764,000	[1]	3,298,000	[1]
Total Past Due	21,055,000		18,743,000
Nonaccrual	79,343,000		65,238,000
Current Loans	6,349,071,000		5,714,900,000
Total LHFI	6,449,469,000		5,798,881,000
LHFI past due 90 days or more	2,800,000		3,300,000
LHFS past due 90 days or more	25,900,000		21,500,000
Percentage of outstanding principal to be repurchased under GNMA optional repurchase program (in hundredths)	100.00%
Amount of delinquent loans repurchased			57,400,000
Repurchase gain included in gain on sales of loans			534,000
Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	248,000		656,000
Past Due 60 - 89 Days	17,000		267,000
Past Due Greater Than 90 Days	60,000	[1]	0	[1]
Total Past Due	325,000		923,000
Nonaccrual	13,867,000		13,327,000
Current Loans	605,685,000		582,639,000
Total LHFI	619,877,000		596,889,000
Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	8,424,000		7,322,000
Past Due 60 - 89 Days	2,428,000		2,115,000
Past Due Greater Than 90 Days	2,367,000	[1]	2,996,000	[1]
Total Past Due	13,219,000		12,433,000
Nonaccrual	25,621,000		21,603,000
Current Loans	1,595,557,000		1,451,528,000
Total LHFI	1,634,397,000		1,485,564,000
Loans Secured by Real Estate [Member] | Nonfarm, Nonresidential Properties [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	1,960,000		1,934,000
Past Due 60 - 89 Days	34,000		110,000
Past Due Greater Than 90 Days	0	[1]	0	[1]
Total Past Due	1,994,000		2,044,000
Nonaccrual	25,717,000		21,809,000
Current Loans	1,525,482,000		1,391,286,000
Total LHFI	1,553,193,000		1,415,139,000
Loans Secured by Real Estate [Member] | Other Real Estate Secured Financing Receivable [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	80,000		2,000
Past Due 60 - 89 Days	0		3,000
Past Due Greater Than 90 Days	0	[1]	0	[1]
Total Past Due	80,000		5,000
Nonaccrual	1,318,000		1,327,000
Current Loans	252,389,000		188,030,000
Total LHFI	253,787,000		189,362,000
Commercial And Industrial Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	2,491,000		809,000
Past Due 60 - 89 Days	306,000		198,000
Past Due Greater Than 90 Days	126,000	[1]	0	[1]
Total Past Due	2,923,000		1,007,000
Nonaccrual	12,104,000		6,286,000
Current Loans	1,255,323,000		1,150,321,000
Total LHFI	1,270,350,000		1,157,614,000
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	1,811,000		1,866,000
Past Due 60 - 89 Days	351,000		146,000
Past Due Greater Than 90 Days	211,000	[1]	302,000	[1]
Total Past Due	2,373,000		2,314,000
Nonaccrual	88,000		151,000
Current Loans	165,503,000		162,843,000
Total LHFI	167,964,000		165,308,000
Other Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	132,000		17,000
Past Due 60 - 89 Days	9,000		0
Past Due Greater Than 90 Days	0	[1]	0	[1]
Total Past Due	141,000		17,000
Nonaccrual	628,000		735,000
Current Loans	949,132,000		788,253,000
Total LHFI	$ 949,901,000		$ 789,005,000

[1] Past due 90 days or more but still accruing interest.