Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Loan Portfolio Held for Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loan Portfolio [Abstract]
Total LHFI	$ 8,747,030	$ 8,569,967
Allowance for loan losses, LHFI	(88,874)	(76,733)
Net LHFI	8,658,156	8,493,234
Construction, Land Development and Other Land [Member] \| Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	1,031,491	987,624
Secured by 1-4 Family Residential Properties [Member] \| Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	1,801,029	1,675,311
Other Real Estate Secured [Member] \| Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	453,687	517,956
Secured by Nonfarm, Nonresidential Properties [Member] \| Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	2,294,289	2,193,823
Commercial and Industrial Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	1,565,922	1,570,345
Consumer Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	182,709	171,918
State and Other Political Subdivision Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	929,178	952,483
Other Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	$ 488,725	$ 500,507

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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