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, 2019	Dec. 31, 2018
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	$ 41,700	$ 55,800
Total LHFI	9,223,668	8,835,868
Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	41,716	55,835
LHFI, Collectively Evaluated For Impairment	9,181,952	8,780,033
Total LHFI	9,223,668	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	1,135,999	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	649	1,552
LHFI, Collectively Evaluated For Impairment	1,135,350	1,055,049
Total LHFI	1,135,999	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	1,820,455	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	5,124	3,963
LHFI, Collectively Evaluated For Impairment	1,815,331	1,821,529
Total LHFI	1,820,455	1,825,492
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	2,442,308	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	6,416	9,476
LHFI, Collectively Evaluated For Impairment	2,435,892	2,211,438
Total LHFI	2,442,308	2,220,914
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	668,667	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	69	248
LHFI, Collectively Evaluated For Impairment	668,598	543,572
Total LHFI	668,667	543,820
Commercial and Industrial Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	1,491,367	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	24,441	30,717
LHFI, Collectively Evaluated For Impairment	1,466,926	1,507,998
Total LHFI	1,491,367	1,538,715
Consumer Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	176,894	182,448
Consumer Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	23	2
LHFI, Collectively Evaluated For Impairment	176,871	182,446
Total LHFI	176,894	182,448
State and Other Political Subdivision Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	978,456	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	974,377	965,223
Total LHFI	978,456	973,818
Other Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	509,522	494,060
Other Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	915	1,282
LHFI, Collectively Evaluated For Impairment	508,607	492,778
Total LHFI	$ 509,522	$ 494,060