Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Loan Portfolio Held for Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Loan Portfolio [Abstract]
Total LHFI	$ 10,174,899	$ 9,824,524
Less allowance for credit losses (ACL), LHFI	104,073	117,306
Net LHFI	10,070,826	9,707,218
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	559,804	514,056
Secured by 1-4 Family Residential Properties [Member] | Other Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	1,382,097	1,216,400
Other Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	509,195	524,732
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	986,163	1,065,964
Other Construction [Member] | Other Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	726,809	794,983
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	2,924,953	2,709,026
Commercial and Industrial Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	1,327,211	1,309,078
Consumer Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	160,802	164,386
State and Other Political Subdivision Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	1,125,186	1,000,776
Other Commercial Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	$ 472,679	$ 525,123