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	Mar. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	$ 52,000	$ 55,800
Total LHFI	8,995,014	8,835,868
Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	52,015	55,835
LHFI, Collectively Evaluated For Impairment	8,942,999	8,780,033
Total LHFI	8,995,014	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	1,209,761	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	695	1,552
LHFI, Collectively Evaluated For Impairment	1,209,066	1,055,049
Total LHFI	1,209,761	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	1,810,872	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	4,249	3,963
LHFI, Collectively Evaluated For Impairment	1,806,623	1,821,529
Total LHFI	1,810,872	1,825,492
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	2,241,072	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	6,938	9,476
LHFI, Collectively Evaluated For Impairment	2,234,134	2,211,438
Total LHFI	2,241,072	2,220,914
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	528,032	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	892	248
LHFI, Collectively Evaluated For Impairment	527,140	543,572
Total LHFI	528,032	543,820
Commercial and Industrial Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	1,558,057	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	29,525	30,717
LHFI, Collectively Evaluated For Impairment	1,528,532	1,507,998
Total LHFI	1,558,057	1,538,715
Consumer Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	176,619	182,448
Consumer Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	31	2
LHFI, Collectively Evaluated For Impairment	176,588	182,446
Total LHFI	176,619	182,448
State and Other Political Subdivision Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	982,626	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	8,449	8,595
LHFI, Collectively Evaluated For Impairment	974,177	965,223
Total LHFI	982,626	973,818
Other Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI	487,975	494,060
Other Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment	1,236	1,282
LHFI, Collectively Evaluated For Impairment	486,739	492,778
Total LHFI	$ 487,975	$ 494,060