Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI (4) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 CreditRiskGrade KeyRatios Loan	Dec. 31, 2013
Financing Receivable Aging [Abstract]
Current	$ 6,349,071,000	$ 5,714,900,000
Nonaccrual	79,343,000	65,238,000
Total LHFI	6,449,469,000	5,798,881,000
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 (in days)	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	605,685,000	582,639,000
Nonaccrual	13,867,000	13,327,000
Total LHFI	619,877,000	596,889,000
Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable Aging [Abstract]
Current	1,595,557,000	1,451,528,000
Nonaccrual	25,621,000	21,603,000
Total LHFI	1,634,397,000	1,485,564,000
Loans Secured by Real Estate [Member] | Nonfarm, Nonresidential Properties [Member]
Financing Receivable Aging [Abstract]
Current	1,525,482,000	1,391,286,000
Nonaccrual	25,717,000	21,809,000
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 Aging [Abstract]
Current	252,389,000	188,030,000
Nonaccrual	1,318,000	1,327,000
Total LHFI	253,787,000	189,362,000
Consumer Loans [Member]
Financing Receivable Aging [Abstract]
Current	165,503,000	162,843,000
Nonaccrual	88,000	151,000
Total LHFI	167,964,000	165,308,000
Other Loans [Member]
Financing Receivable Aging [Abstract]
Current	949,132,000	788,253,000
Nonaccrual	628,000	735,000
Total LHFI	949,901,000	789,005,000
Commercial Portfolio Segment [Member]
Financing Receivable [Abstract]
Financing receivable commercial	4,708,378,000	4,213,735,000
Commercial Portfolio Segment [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	4,519,970,000	3,982,761,000
Commercial Portfolio Segment [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	22,663,000	36,313,000
Commercial Portfolio Segment [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	164,053,000	190,743,000
Commercial Portfolio Segment [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	1,692,000	3,918,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member]
Financing Receivable [Abstract]
Financing receivable commercial	556,641,000	545,691,000
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	518,944,000	493,380,000
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	479,000	4,383,000
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	37,022,000	47,610,000
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	196,000	318,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable [Abstract]
Financing receivable commercial	134,551,000	128,068,000
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	125,203,000	119,640,000
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	1,652,000	479,000
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	7,483,000	7,839,000
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	213,000	110,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Nonfarm, Nonresidential Properties [Member]
Financing Receivable [Abstract]
Financing receivable commercial	1,552,318,000	1,413,692,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Nonfarm, Nonresidential Properties [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	1,462,226,000	1,313,470,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Nonfarm, Nonresidential Properties [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	8,431,000	12,620,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Nonfarm, Nonresidential Properties [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	81,661,000	87,203,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Nonfarm, Nonresidential Properties [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	399,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured Financing Receivable [Member]
Financing Receivable [Abstract]
Financing receivable commercial	251,380,000	185,942,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured Financing Receivable [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	246,099,000	178,951,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured Financing Receivable [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	306,000	0
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured Financing Receivable [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	4,975,000	6,756,000
Commercial Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured Financing Receivable [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	235,000
Commercial Portfolio Segment [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial	1,270,344,000	1,157,596,000
Commercial Portfolio Segment [Member] | Commercial and Industrial Loans [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	1,239,247,000	1,099,429,000
Commercial Portfolio Segment [Member] | Commercial and Industrial Loans [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	4,245,000	18,771,000
Commercial Portfolio Segment [Member] | Commercial and Industrial Loans [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	26,133,000	37,209,000
Commercial Portfolio Segment [Member] | Commercial and Industrial Loans [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	719,000	2,187,000
Commercial Portfolio Segment [Member] | Consumer Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	496,000
Commercial Portfolio Segment [Member] | Consumer Loans [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	0	496,000
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] | Other Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial	943,144,000	782,250,000
Commercial Portfolio Segment [Member] | Other Loans [Member] | Pass [Member]
Financing Receivable [Abstract]
Financing receivable commercial	928,251,000	777,395,000
Commercial Portfolio Segment [Member] | Other Loans [Member] | Special Mention [Member]
Financing Receivable [Abstract]
Financing receivable commercial	7,550,000	60,000
Commercial Portfolio Segment [Member] | Other Loans [Member] | Substandard [Member]
Financing Receivable [Abstract]
Financing receivable commercial	6,779,000	4,126,000
Commercial Portfolio Segment [Member] | Other Loans [Member] | Doubtful [Member]
Financing Receivable [Abstract]
Financing receivable commercial	564,000	669,000
Consumer Portfolio Segment [Member]
Financing Receivable Aging [Abstract]
Current	1,703,795,000	1,552,447,000
Past Due 30 - 89 Days	12,795,000	11,095,000
Past Due Greater Than 90 Days	2,638,000	3,298,000
Nonaccrual	21,863,000	18,306,000
Subtotal	1,741,091,000	1,585,146,000
Total LHFI	6,449,469,000	5,798,881,000
Consumer Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member]
Financing Receivable Aging [Abstract]
Current	62,897,000	50,850,000
Past Due 30 - 89 Days	199,000	131,000
Past Due Greater Than 90 Days	59,000	0
Nonaccrual	81,000	217,000
Subtotal	63,236,000	51,198,000
Total LHFI	619,877,000	596,889,000
Consumer Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable Aging [Abstract]
Current	1,465,355,000	1,327,624,000
Past Due 30 - 89 Days	10,429,000	8,937,000
Past Due Greater Than 90 Days	2,367,000	2,996,000
Nonaccrual	21,695,000	17,939,000
Subtotal	1,499,846,000	1,357,496,000
Total LHFI	1,634,397,000	1,485,564,000
Consumer Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Nonfarm, Nonresidential Properties [Member]
Financing Receivable Aging [Abstract]
Current	875,000	1,439,000
Past Due 30 - 89 Days	0	8,000
Past Due Greater Than 90 Days	0	0
Nonaccrual	0	0
Subtotal	875,000	1,447,000
Total LHFI	1,553,193,000	1,415,139,000
Consumer Portfolio Segment [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured Financing Receivable [Member]
Financing Receivable Aging [Abstract]
Current	2,407,000	3,418,000
Past Due 30 - 89 Days	0	2,000
Past Due Greater Than 90 Days	0	0
Nonaccrual	0	0
Subtotal	2,407,000	3,420,000
Total LHFI	253,787,000	189,362,000
Consumer Portfolio Segment [Member] | Commercial and Industrial Loans [Member]
Financing Receivable Aging [Abstract]
Current	0	13,000
Past Due 30 - 89 Days	5,000	5,000
Past Due Greater Than 90 Days	1,000	0
Nonaccrual	0	0
Subtotal	6,000	18,000
Total LHFI	1,270,350,000	1,157,614,000
Consumer Portfolio Segment [Member] | Consumer Loans [Member]
Financing Receivable Aging [Abstract]
Current	165,504,000	162,348,000
Past Due 30 - 89 Days	2,162,000	2,012,000
Past Due Greater Than 90 Days	211,000	302,000
Nonaccrual	87,000	150,000
Subtotal	167,964,000	164,812,000
Total LHFI	167,964,000	165,308,000
Consumer Portfolio Segment [Member] | Other Loans [Member]
Financing Receivable Aging [Abstract]
Current	6,757,000	6,755,000
Past Due 30 - 89 Days	0	0
Past Due Greater Than 90 Days	0	0
Nonaccrual	0	0
Subtotal	6,757,000	6,755,000
Total LHFI	$ 949,901,000	$ 789,005,000