Loans Acquired - Textual (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Oct. 19, 2017	Oct. 18, 2017	Jun. 30, 2017	May 15, 2017	May 14, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 11,858,195,000		$ 11,858,195,000			$ 10,779,685,000
Allowance for loan losses	56,703,000	$ 43,108,000	56,703,000	$ 43,108,000	$ 53,776,000	42,086,000			$ 41,770,000			$ 37,240,000
Provision for loan losses	10,345,000	5,462,000	28,528,000	16,792,000
Loans acquired, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	3,734,921,000		3,734,921,000			5,074,076,000
Allowance for loan losses	1,345,000	391,000	1,345,000	391,000	$ 2,044,000	418,000						$ 954,000
Provision for loan losses	$ 0	$ 0	$ 1,837,000	$ 1,464,000
OKSB Merger
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans						1,985,000,000
Gross value of loans acquired						2,021,000,000	$ 1,996,453,000	$ 2,039,524,000
Loans acquired discount						36,300,000
OKSB Merger | Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans						11,400,000
Gross value of loans acquired						18,100,000
Loans acquired discount						6,700,000
First Texas Merger
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans						2,208,000,000
Gross value of loans acquired						2,246,000,000	$ 2,208,378,000	$ 2,246,212,000
Loans acquired discount						$ 37,800,000
Hardeman County Investment Company, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans									249,200,000
Gross value of loans acquired									254,200,000	$ 251,649,000	$ 257,641,000
Loans acquired discount									5,000,000
Hardeman County Investment Company, Inc. | Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans									2,400,000
Gross value of loans acquired									3,400,000
Loans acquired discount									$ 990,000