Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI (4) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015 CreditRiskGrade LoanPool KeyRatio	Dec. 31, 2014
Financing Receivable Aging [Abstract]
Current	$ 6,318,849,000	$ 6,349,071,000
Nonaccrual	77,008,000	79,343,000
Total LHFI	6,413,876,000	6,449,469,000
Credit amount used as baseline in evaluating residential real estate development and non-owner occupied commercial real estate for loan policy	1,000,000
Number of days used as baseline in evaluating collateral documentation exceptions for loan policy	90 days
Loan amount used as baseline in credit quality review for loan policy	100,000
Number of key quality ratios	6
Number of individual credit risk grades	10
Number of primary commercial loan groups	9
Minimum [Member]
Financing Receivable Aging [Abstract]
Period to conduct asset review	6 months
Maximum [Member]
Financing Receivable Aging [Abstract]
Period to conduct asset review	18 months
Loans Secured by Real Estate [Member] | Construction Land Development and Other Land [Member]
Financing Receivable Aging [Abstract]
Current	670,117,000	605,685,000
Nonaccrual	20,134,000	13,867,000
Total LHFI	691,657,000	619,877,000
Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable Aging [Abstract]
Current	1,579,405,000	1,595,557,000
Nonaccrual	23,620,000	25,621,000
Total LHFI	1,613,993,000	1,634,397,000
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member]
Financing Receivable Aging [Abstract]
Current	1,496,573,000	1,525,482,000
Nonaccrual	17,887,000	25,717,000
Total LHFI	1,516,895,000	1,553,193,000
Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member]
Financing Receivable Aging [Abstract]
Current	232,516,000	252,389,000
Nonaccrual	723,000	1,318,000
Total LHFI	233,322,000	253,787,000
Consumer Loans [Member]
Financing Receivable Aging [Abstract]
Current	160,881,000	165,503,000
Nonaccrual	96,000	88,000
Total LHFI	161,535,000	167,964,000
State and Other Political Subdivision Loans [Member]
Financing Receivable Aging [Abstract]
Current	614,330,000	602,727,000
Nonaccrual	0	0
Total LHFI	614,330,000	602,727,000
Other Loans [Member]
Financing Receivable Aging [Abstract]
Current	351,331,000	346,405,000
Nonaccrual	491,000	628,000
Total LHFI	353,356,000	347,174,000
Commercial Portfolio Segment [Member]
Financing Receivable [Abstract]
Financing receivable commercial	4,694,480,000	4,708,378,000
Commercial Portfolio Segment [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	4,522,438,000	4,519,970,000
Commercial Portfolio Segment [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	12,504,000	22,663,000
Commercial Portfolio Segment [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	157,720,000	164,053,000
Commercial Portfolio Segment [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	1,818,000	1,692,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Construction Land Development and Other Land [Member]
Financing Receivable [Abstract]
Financing receivable commercial	624,463,000	556,641,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Construction Land Development and Other Land [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	581,894,000	518,944,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Construction Land Development and Other Land [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	554,000	479,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Construction Land Development and Other Land [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	41,551,000	37,022,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Construction Land Development and Other Land [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	464,000	196,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable [Abstract]
Financing receivable commercial	131,837,000	134,551,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	124,564,000	125,203,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	477,000	1,652,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	6,513,000	7,483,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	283,000	213,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member]
Financing Receivable [Abstract]
Financing receivable commercial	1,516,057,000	1,552,318,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	1,449,377,000	1,462,226,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	993,000	8,431,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	65,687,000	81,661,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	0
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member]
Financing Receivable [Abstract]
Financing receivable commercial	231,371,000	251,380,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	225,962,000	246,099,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	276,000	306,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	5,133,000	4,975,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	0
Commercial Portfolio Segment [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial	1,228,781,000	1,270,344,000
Commercial Portfolio Segment [Member] | Commercial and Industrial Loans [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	1,197,042,000	1,239,247,000
Commercial Portfolio Segment [Member] | Commercial and Industrial Loans [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	2,654,000	4,245,000
Commercial Portfolio Segment [Member] | Commercial and Industrial Loans [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	28,432,000	26,133,000
Commercial Portfolio Segment [Member] | Commercial and Industrial Loans [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	653,000	719,000
Commercial Portfolio Segment [Member] | Consumer Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	0
Commercial Portfolio Segment [Member] | Consumer Loans [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	0
Commercial Portfolio Segment [Member] | Consumer Loans [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	0
Commercial Portfolio Segment [Member] | Consumer Loans [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	0
Commercial Portfolio Segment [Member] | Consumer Loans [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	0
Commercial Portfolio Segment [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial	614,330,000	602,727,000
Commercial Portfolio Segment [Member] | State and Other Political Subdivision Loans [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	597,230,000	589,653,000
Commercial Portfolio Segment [Member] | State and Other Political Subdivision Loans [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	7,550,000	7,550,000
Commercial Portfolio Segment [Member] | State and Other Political Subdivision Loans [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	9,550,000	5,524,000
Commercial Portfolio Segment [Member] | State and Other Political Subdivision Loans [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	0
Commercial Portfolio Segment [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial	347,641,000	340,417,000
Commercial Portfolio Segment [Member] | Other Loans [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	346,369,000	338,598,000
Commercial Portfolio Segment [Member] | Other Loans [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	0
Commercial Portfolio Segment [Member] | Other Loans [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	854,000	1,255,000
Commercial Portfolio Segment [Member] | Other Loans [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	418,000	564,000
Consumer Portfolio Segment [Member]
Financing Receivable Aging [Abstract]
Current	1,686,997,000	1,703,795,000
Past Due 30-89 Days	10,412,000	12,795,000
Past Due 90 Days or More	1,413,000	2,638,000
Nonaccrual	20,574,000	21,863,000
Subtotal	1,719,396,000	1,741,091,000
Total LHFI	6,413,876,000	6,449,469,000
Consumer Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Construction Land Development and Other Land [Member]
Financing Receivable Aging [Abstract]
Current	66,852,000	62,897,000
Past Due 30-89 Days	220,000	199,000
Past Due 90 Days or More	0	59,000
Nonaccrual	122,000	81,000
Subtotal	67,194,000	63,236,000
Total LHFI	691,657,000	619,877,000
Consumer Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable Aging [Abstract]
Current	1,451,991,000	1,465,355,000
Past Due 30-89 Days	8,601,000	10,429,000
Past Due 90 Days or More	1,207,000	2,367,000
Nonaccrual	20,357,000	21,695,000
Subtotal	1,482,156,000	1,499,846,000
Total LHFI	1,613,993,000	1,634,397,000
Consumer Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member]
Financing Receivable Aging [Abstract]
Current	838,000	875,000
Past Due 30-89 Days	0	0
Past Due 90 Days or More	0	0
Nonaccrual	0	0
Subtotal	838,000	875,000
Total LHFI	1,516,895,000	1,553,193,000
Consumer Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member]
Financing Receivable Aging [Abstract]
Current	1,948,000	2,407,000
Past Due 30-89 Days	3,000	0
Past Due 90 Days or More	0	0
Nonaccrual	0	0
Subtotal	1,951,000	2,407,000
Total LHFI	233,322,000	253,787,000
Consumer Portfolio Segment [Member] | Commercial and Industrial Loans [Member]
Financing Receivable Aging [Abstract]
Current	3,000	0
Past Due 30-89 Days	4,000	5,000
Past Due 90 Days or More	0	1,000
Nonaccrual	0	0
Subtotal	7,000	6,000
Total LHFI	1,228,788,000	1,270,350,000
Consumer Portfolio Segment [Member] | Consumer Loans [Member]
Financing Receivable Aging [Abstract]
Current	160,882,000	165,504,000
Past Due 30-89 Days	391,000	2,162,000
Past Due 90 Days or More	167,000	211,000
Nonaccrual	95,000	87,000
Subtotal	161,535,000	167,964,000
Total LHFI	161,535,000	167,964,000
Consumer Portfolio Segment [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable Aging [Abstract]
Current	0	0
Past Due 30-89 Days	0	0
Past Due 90 Days or More	0	0
Nonaccrual	0	0
Subtotal	0	0
Total LHFI	614,330,000	602,727,000
Consumer Portfolio Segment [Member] | Other Loans [Member]
Financing Receivable Aging [Abstract]
Current	4,483,000	6,757,000
Past Due 30-89 Days	1,193,000	0
Past Due 90 Days or More	39,000	0
Nonaccrual	0	0
Subtotal	5,715,000	6,757,000
Total LHFI	$ 353,356,000	$ 347,174,000