Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Loan Portfolio Held for Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Loan Portfolio [Abstract]
Total LHFI	$ 8,995,014	$ 8,835,868
Less allowance for loan losses, LHFI	79,005	79,290
Net LHFI	8,916,009	8,756,578
Construction, Land Development and Other Land [Member] \| Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	1,209,761	1,056,601
Secured by 1-4 Family Residential Properties [Member] \| Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	1,810,872	1,825,492
Other Real Estate Secured [Member] \| Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	528,032	543,820
Secured by Nonfarm, Nonresidential Properties [Member] \| Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	2,241,072	2,220,914
Commercial and Industrial Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	1,558,057	1,538,715
Consumer Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	176,619	182,448
State and Other Political Subdivision Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	982,626	973,818
Other Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	$ 487,975	$ 494,060

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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