CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
NET INCOME (LOSS)	$ 34,136,000	$ (972,000)	$ 15,439,000
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization and accretion of premiums and discounts on securities, net	1,764,000	821,000	1,122,000
Accretion of loan discounts, net	(4,755,000)	(8,358,000)	(4,806,000)
Amortization of deposit premiums	(995,000)	(2,362,000)	(1,747,000)
Amortization of core deposit intangibles	1,472,000	1,587,000	847,000
Amortization and accretion of borrowing and debt valuation allowance, net	420,000	289,000	40,000
Amortization of issuance costs of subordinated notes	145,000	9,000	0
Provision for loan losses	14,360,000	64,700,000	17,864,000
Depreciation	2,044,000	2,388,000	1,849,000
Net loss (gain) on sale of available-for-sale securities	38,000	0	(772,000)
Net loss (gain) on sale of real estate acquired by foreclosure	147,000	563,000	81,000
Net loss (gain) on loans held for sale	2,308,000	1,404,000	(405,000)
Net gain on sale of guaranteed portion of loans	(5,755,000)	(3,343,000)	(3,484,000)
Originations of loans held for sale	0	(1,094,000)	(17,656,000)
Proceeds from sales of and principal collected on loans held for sale	16,845,000	21,488,000	19,875,000
Writedown of real estate acquired by foreclosure	304,000	1,048,000	530,000
Deferred income tax expense (benefit)	5,243,000	8,361,000	(3,409,000)
Stock-based compensation expense	4,880,000	1,587,000	707,000
(Increase) decrease in accrued interest receivable and other assets, net	12,775,000	(16,520,000)	2,208,000
Increase (decrease)in accrued interest payable and other liabilities, net	3,658,000	1,838,000	(1,379,000)
Net cash provided (used in) by operating activities	89,034,000	73,434,000	26,904,000
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from the maturities or calls and paydowns of available-for-sale securities	72,082,000	110,423,000	51,690,000
Proceeds from the sale of available-for-sale securities	349,374,000	0	109,444,000
Purchases of available-for-sale securities	(847,078,000)	(100,335,000)	(53,046,000)
Proceeds from the maturities or calls and paydowns of held-to-maturity securities	6,276,000	8,507,000	10,192,000
Purchases of held-to-maturity securities	0	(1,963,000)	(1,788,000)
Proceeds from sales of guaranteed portion of loans	56,102,000	39,109,000	38,201,000
Proceeds from sales of real estate acquired by foreclosure	3,967,000	8,794,000	2,587,000
Purchases of Federal Home Loan Bank of Dallas stock, net of redemptions	(7,886,000)	6,084,000	359,000
Purchases of Federal Reserve Bank stock	(748,000)	(3,747,000)	(34,000)
Purchase of Bank-Owned Life Insurance	(20,000,000)	0	0
Net increase in loans held for investment and held for sale	(154,951,000)	(115,148,000)	(279,756,000)
Investment in construction (sale of) premises and purchases of other fixed assets	4,898,000	(326,000)	(1,349,000)
Net cash and cash equivalents received (paid) in connection with acquisitions	0	0	4,275,000
Net cash (used by) provided by investing activities	(537,964,000)	(48,602,000)	(119,225,000)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net increase in deposit accounts	23,438,000	276,314,000	170,236,000
Net (decrease) increase in securities sold under agreements to repurchase	1,680,000	420,000	(1,532,000)
Net proceeds from (repayment of) other short-term borrowed funds	175,000,000	(64,250,000)	(113,250,000)
Proceeds from other long-term borrowed funds	0	50,000,000	100,000,000
Repayment of other long-term borrowed funds	0	(58,884,000)	(8,684,000)
Proceeds from issuance of common stock due to exercise of stock options	486,000	3,046,000	1,534,000
Proceeds from issuance of subordinated notes, net	0	33,876,000	0
Purchase of treasury stock	0	(1,253,000)	0
Net cash provided by financing activities	200,604,000	239,269,000	148,304,000
NET INCREASE IN CASH AND CASH EQUIVALENTS	(248,326,000)	264,101,000	55,983,000
CASH AND CASH EQUIVALENTS:
Beginning of year	389,007,000	124,906,000	68,923,000
End of year	140,681,000	389,007,000	124,906,000
NONCASH ACTIVITIES:
Noncash investing and financing activities - acquisition of real estate through foreclosure of collateral	5,270,000	3,503,000	0
Transfer of loans to held-for-sale	15,185,000	45,403,000	0
Stock issued in connection with the Patriot Bancshares acquisition	0	0	123,743,000
SUPPLEMENTAL INFORMATION:
Interest paid	31,179,000	23,473,000	11,549,000
Income taxes paid	$ 16,200,000	$ 11,700,000	$ 12,200,000