Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI (5) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	$ 12,471,000		$ 15,146,000
Past Due 60 - 89 Days	4,135,000		3,145,000
Past Due 90 Days or More	1,413,000	[1]	2,764,000	[1]
Total Past Due	18,019,000		21,055,000
Nonaccrual	77,008,000		79,343,000
Current Loans	6,318,849,000		6,349,071,000
Total LHFI	6,413,876,000		6,449,469,000
LHFI past due 90 days or more	1,400,000		2,800,000
LHFS past due 90 days or more	7,600,000		25,900,000
Percentage of outstanding principal to be repurchased under GNMA optional repurchase program (in hundredths)	100.00%
Amount of delinquent loans repurchased	28,500,000
Repurchase gain included in gain on sales of loans	304,000
Loans Secured by Real Estate [Member] | Construction Land Development and Other Land [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	320,000		248,000
Past Due 60 - 89 Days	1,086,000		17,000
Past Due 90 Days or More	0	[1]	60,000	[1]
Total Past Due	1,406,000		325,000
Nonaccrual	20,134,000		13,867,000
Current Loans	670,117,000		605,685,000
Total LHFI	691,657,000		619,877,000
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	7,872,000		8,424,000
Past Due 60 - 89 Days	1,889,000		2,428,000
Past Due 90 Days or More	1,207,000	[1]	2,367,000	[1]
Total Past Due	10,968,000		13,219,000
Nonaccrual	23,620,000		25,621,000
Current Loans	1,579,405,000		1,595,557,000
Total LHFI	1,613,993,000		1,634,397,000
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	1,640,000		1,960,000
Past Due 60 - 89 Days	795,000		34,000
Past Due 90 Days or More	0	[1]	0	[1]
Total Past Due	2,435,000		1,994,000
Nonaccrual	17,887,000		25,717,000
Current Loans	1,496,573,000		1,525,482,000
Total LHFI	1,516,895,000		1,553,193,000
Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	83,000		80,000
Past Due 60 - 89 Days	0		0
Past Due 90 Days or More	0	[1]	0	[1]
Total Past Due	83,000		80,000
Nonaccrual	723,000		1,318,000
Current Loans	232,516,000		252,389,000
Total LHFI	233,322,000		253,787,000
Commercial and Industrial Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	947,000		2,491,000
Past Due 60 - 89 Days	88,000		306,000
Past Due 90 Days or More	0	[1]	126,000	[1]
Total Past Due	1,035,000		2,923,000
Nonaccrual	14,057,000		12,104,000
Current Loans	1,213,696,000		1,255,323,000
Total LHFI	1,228,788,000		1,270,350,000
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	230,000		1,811,000
Past Due 60 - 89 Days	161,000		351,000
Past Due 90 Days or More	167,000	[1]	211,000	[1]
Total Past Due	558,000		2,373,000
Nonaccrual	96,000		88,000
Current Loans	160,881,000		165,503,000
Total LHFI	161,535,000		167,964,000
State and Other Political Subdivision Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	0		0
Past Due 60 - 89 Days	0		0
Past Due 90 Days or More	0	[1]	0	[1]
Total Past Due	0		0
Nonaccrual	0		0
Current Loans	614,330,000		602,727,000
Total LHFI	614,330,000		602,727,000
Other Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Past Due 30 - 59 Days	1,379,000		132,000
Past Due 60 - 89 Days	116,000		9,000
Past Due 90 Days or More	39,000	[1]	0	[1]
Total Past Due	1,534,000		141,000
Nonaccrual	491,000		628,000
Current Loans	351,331,000		346,405,000
Total LHFI	$ 353,356,000		$ 347,174,000

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