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	Jun. 30, 2018	Dec. 31, 2017
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	$ 45,600	$ 48,900
Total LHFI	8,678,983	8,569,967
Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	45,581	48,936
LHFI, Collectively Evaluated For Impairment	8,633,402	8,521,031
Total LHFI	8,678,983	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	1,038,745	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	1,766	1,405
LHFI, Collectively Evaluated For Impairment	1,036,979	986,219
Total LHFI	1,038,745	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	1,742,496	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	4,615	4,736
LHFI, Collectively Evaluated For Impairment	1,737,881	1,670,575
Total LHFI	1,742,496	1,675,311
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	2,321,734	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	6,518	10,423
LHFI, Collectively Evaluated For Impairment	2,315,216	2,183,400
Total LHFI	2,321,734	2,193,823
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	397,538	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	397,538	517,956
Total LHFI	397,538	517,956
Commercial and Industrial Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	1,572,764	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	31,572	31,799
LHFI, Collectively Evaluated For Impairment	1,541,192	1,538,546
Total LHFI	1,572,764	1,570,345
Consumer Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	175,261	171,918
Consumer Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	4	17
LHFI, Collectively Evaluated For Impairment	175,257	171,901
Total LHFI	175,261	171,918
State and Other Political Subdivision Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	925,452	952,483
State and Other Political Subdivision Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Collectively Evaluated For Impairment	925,452	952,483
Total LHFI	925,452	952,483
Other Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	504,993	500,507
Other Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	1,106	556
LHFI, Collectively Evaluated For Impairment	503,887	499,951
Total LHFI	$ 504,993	$ 500,507