Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Loan Portfolio Held for Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Loan Portfolio [Abstract]
Total LHFI	$ 9,983,704	$ 9,824,524
Less allowance for credit losses (ACL), LHFI	109,191	117,306
Net LHFI	9,874,513	9,707,218
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	542,902	514,056
Secured by 1-4 Family Residential Properties [Member] | Other Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	1,233,050	1,216,400
Other Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	509,732	524,732
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	1,135,005	1,065,964
Other Construction [Member] | Other Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	799,186	794,983
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	2,799,195	2,709,026
Commercial and Industrial Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	1,323,277	1,309,078
Consumer Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	155,356	164,386
State and Other Political Subdivision Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	1,036,694	1,000,776
Other Commercial Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	$ 449,307	$ 525,123