Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Summary of Loan Type Related to Each Balance in Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	$ 52,500	$ 48,900
Total LHFI	8,747,030	8,569,967
Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	52,455	48,936
LHFI, Collectively Evaluated For Impairment	8,694,575	8,521,031
Total LHFI	8,747,030	8,569,967
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	1,031,491	987,624
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	1,572	1,405
LHFI, Collectively Evaluated For Impairment	1,029,919	986,219
Total LHFI	1,031,491	987,624
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	1,801,029	1,675,311
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	3,740	4,736
LHFI, Collectively Evaluated For Impairment	1,797,289	1,670,575
Total LHFI	1,801,029	1,675,311
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	2,294,289	2,193,823
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	3,370	10,423
LHFI, Collectively Evaluated For Impairment	2,290,919	2,183,400
Total LHFI	2,294,289	2,193,823
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	453,687	517,956
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Collectively Evaluated For Impairment	453,687	517,956
Total LHFI	453,687	517,956
Commercial and Industrial Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	1,565,922	1,570,345
Commercial and Industrial Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	33,702	31,799
LHFI, Collectively Evaluated For Impairment	1,532,220	1,538,546
Total LHFI	1,565,922	1,570,345
Consumer Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	182,709	171,918
Consumer Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	3	17
LHFI, Collectively Evaluated For Impairment	182,706	171,901
Total LHFI	182,709	171,918
State and Other Political Subdivision Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	929,178	952,483
State and Other Political Subdivision Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	8,762
LHFI, Collectively Evaluated For Impairment	920,416	952,483
Total LHFI	929,178	952,483
Other Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	488,725	500,507
Other Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	1,306	556
LHFI, Collectively Evaluated For Impairment	487,419	499,951
Total LHFI	$ 488,725	$ 500,507