Loans Acquired - Summary of the Non-covered Impaired Loans Acquired in Acquisitions - Loans Acquired, Not Covered By FDIC Loss Share (Details) - Loans Acquired, Not Covered By FDIC Loss Share - USD ($) $ in Thousands	Oct. 19, 2017	May 15, 2017	Sep. 09, 2016
OKSB Merger
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition	$ 2,021,388
Non-accretable difference (expected losses and foregone interest)	0
Cash flows expected to be collected at acquisition	2,021,388
Accretable yield	(36,340)
Basis in acquired loans at acquisition	1,985,048
OKSB Merger | Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition	18,136
Non-accretable difference (expected losses and foregone interest)	(6,731)
Cash flows expected to be collected at acquisition	11,405
Accretable yield	0
Basis in acquired loans at acquisition	11,405
First Texas Merger
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition	2,246,212
Non-accretable difference (expected losses and foregone interest)	0
Cash flows expected to be collected at acquisition	2,246,212
Accretable yield	(37,834)
Basis in acquired loans at acquisition	2,208,378
First Texas Merger | Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition	0
Non-accretable difference (expected losses and foregone interest)	0
Cash flows expected to be collected at acquisition	0
Accretable yield	0
Basis in acquired loans at acquisition	$ 0
Hardeman County Investment Company, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition		$ 254,189
Non-accretable difference (expected losses and foregone interest)		0
Cash flows expected to be collected at acquisition		254,189
Accretable yield		(5,002)
Basis in acquired loans at acquisition		249,187
Hardeman County Investment Company, Inc. | Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition		3,452
Non-accretable difference (expected losses and foregone interest)		(990)
Cash flows expected to be collected at acquisition		2,462
Accretable yield		0
Basis in acquired loans at acquisition		$ 2,462
Citizens National Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition			$ 348,756
Non-accretable difference (expected losses and foregone interest)			0
Cash flows expected to be collected at acquisition			348,756
Accretable yield			(8,663)
Basis in acquired loans at acquisition			340,093
Citizens National Bank | Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition			1,605
Non-accretable difference (expected losses and foregone interest)			(848)
Cash flows expected to be collected at acquisition			757
Accretable yield			0
Basis in acquired loans at acquisition			$ 757