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, 2019	Dec. 31, 2018
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	$ 51,100	$ 55,800
Total LHFI	9,116,759	8,835,868
Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	51,069	55,835
LHFI, Collectively Evaluated For Impairment	9,065,690	8,780,033
Total LHFI	9,116,759	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	1,111,297	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	672	1,552
LHFI, Collectively Evaluated For Impairment	1,110,625	1,055,049
Total LHFI	1,111,297	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	1,818,126	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	4,698	3,963
LHFI, Collectively Evaluated For Impairment	1,813,428	1,821,529
Total LHFI	1,818,126	1,825,492
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	2,326,312	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	6,773	9,476
LHFI, Collectively Evaluated For Impairment	2,319,539	2,211,438
Total LHFI	2,326,312	2,220,914
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	635,839	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		248
LHFI, Collectively Evaluated For Impairment	635,839	543,572
Total LHFI	635,839	543,820
Commercial and Industrial Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	1,533,318	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	29,389	30,717
LHFI, Collectively Evaluated For Impairment	1,503,929	1,507,998
Total LHFI	1,533,318	1,538,715
Consumer Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	176,133	182,448
Consumer Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	30	2
LHFI, Collectively Evaluated For Impairment	176,103	182,446
Total LHFI	176,133	182,448
State and Other Political Subdivision Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	982,187	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	8,303	8,595
LHFI, Collectively Evaluated For Impairment	973,884	965,223
Total LHFI	982,187	973,818
Other Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	533,547	494,060
Other Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	1,204	1,282
LHFI, Collectively Evaluated For Impairment	532,343	492,778
Total LHFI	$ 533,547	$ 494,060