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	Dec. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	$ 41,200	$ 55,800
Total LHFI	9,335,628	8,835,868
Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	41,217	55,835
LHFI, Collectively Evaluated For Impairment	9,294,411	8,780,033
Total LHFI	9,335,628	8,835,868
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	1,162,791	1,056,601
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	626	1,552
LHFI, Collectively Evaluated For Impairment	1,162,165	1,055,049
Total LHFI	1,162,791	1,056,601
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	1,855,913	1,825,492
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	5,464	3,963
LHFI, Collectively Evaluated For Impairment	1,850,449	1,821,529
Total LHFI	1,855,913	1,825,492
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	2,475,245	2,220,914
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	6,717	9,476
LHFI, Collectively Evaluated For Impairment	2,468,528	2,211,438
Total LHFI	2,475,245	2,220,914
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	724,480	543,820
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, Individually Evaluated For Impairment	68	248
LHFI, Collectively Evaluated For Impairment	724,412	543,572
Total LHFI	724,480	543,820
Commercial and Industrial Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	1,477,896	1,538,715
Commercial and Industrial Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	23,458	30,717
LHFI, Collectively Evaluated For Impairment	1,454,438	1,507,998
Total LHFI	1,477,896	1,538,715
Consumer Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	175,738	182,448
Consumer Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	21	2
LHFI, Collectively Evaluated For Impairment	175,717	182,446
Total LHFI	175,738	182,448
State and Other Political Subdivision Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	967,944	973,818
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	4,079	8,595
LHFI, Collectively Evaluated For Impairment	963,865	965,223
Total LHFI	967,944	973,818
Other Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	495,621	494,060
Other Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	784	1,282
LHFI, Collectively Evaluated For Impairment	494,837	492,778
Total LHFI	$ 495,621	$ 494,060