Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Change in Allowance for Loan Losses (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				$ 76,733
Provision for loan losses, LHFI		$ 8,673	$ 3,672	15,801	$ 9,355
Balance at end of period		88,874		88,874
Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		83,566	76,184	76,733	71,265
Transfers	[1]	772		1,554
Loans charged-off		(7,017)	(2,752)	(12,980)	(9,072)
Recoveries		2,880	3,228	7,766	8,784
Net (charge-offs) recoveries		(4,137)	476	(5,214)	(288)
Provision for loan losses, LHFI		8,673	3,672	15,801	9,355
Balance at end of period		88,874	80,332	88,874	80,332
Allowance for Loan Losses, LHFI [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				7,865	9,085
Transfers	[2]			584
Loans charged-off				(123)	(79)
Recoveries				1,039	1,257
Provision for loan losses, LHFI				(951)	(1,615)
Balance at end of period		8,414	8,648	8,414	8,648
Allowance for Loan Losses, LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				10,874	10,347
Transfers	[2]			182
Loans charged-off				(1,435)	(753)
Recoveries				439	1,429
Provision for loan losses, LHFI				(752)	(743)
Balance at end of period		9,308	10,280	9,308	10,280
Allowance for Loan Losses, LHFI [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				23,428	20,967
Transfers	[2]			446
Loans charged-off				(1,117)	(50)
Recoveries				120	289
Provision for loan losses, LHFI				1,982	7,542
Balance at end of period		24,859	28,748	24,859	28,748
Allowance for Loan Losses, LHFI [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				2,790	2,263
Transfers	[2]			291
Recoveries				17	31
Provision for loan losses, LHFI				(149)	1,111
Balance at end of period		2,949	3,405	2,949	3,405
Allowance for Loan Losses, LHFI [Member] | Commercial and Industrial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				22,851	22,011
Transfers	[2]			46
Loans charged-off				(4,658)	(2,624)
Recoveries				2,285	1,813
Provision for loan losses, LHFI				12,793	(1,420)
Balance at end of period		33,317	19,780	33,317	19,780
Allowance for Loan Losses, LHFI [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				3,470	3,241
Transfers	[2]			5
Loans charged-off				(1,585)	(1,809)
Recoveries				1,545	1,427
Provision for loan losses, LHFI				(41)	1,597
Balance at end of period		3,394	4,456	3,394	4,456
Allowance for Loan Losses, LHFI [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				789	859
Provision for loan losses, LHFI				404	(48)
Balance at end of period		1,193	811	1,193	811
Allowance for Loan Losses, LHFI [Member] | Other Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				4,666	2,492
Loans charged-off				(4,062)	(3,757)
Recoveries				2,321	2,538
Provision for loan losses, LHFI				2,515	2,931
Balance at end of period		$ 5,440	$ 4,204	$ 5,440	$ 4,204

[1] The allowance for loan losses balance related to the remaining loans acquired in the Bay Bank & Trust Company (Bay Bank) merger on March 16, 2012, which were transferred from acquired impaired loans to LHFI during the second quarter of 2018, and the remaining loans acquired in the Heritage Banking Group (Heritage) acquisition on April 15, 2011 and the Reliance merger on April 7, 2017, which were transferred from acquired impaired loans to LHFI during the third quarter of 2018.

[2] The allowance for loan losses balance related to the remaining loans acquired in the Bay Bank merger on March 16, 2012, which were transferred from acquired impaired loans to LHFI during the second quarter of 2018, and the remaining loans acquired in the Heritage acquisition on April 15, 2011 and the Reliance merger on April 7, 2017, which were transferred from acquired impaired loans to LHFI during the third quarter of 2018.