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. 31, 2019	Apr. 12, 2019	Oct. 19, 2017	May 15, 2017
Landrum Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition	$ 2,033,315
Non-accretable difference (expected losses and foregone interest)	0
Cash flows expected to be collected at acquisition	2,033,315
Accretable yield	(38,555)
Basis in acquired loans at acquisition	1,994,760
Landrum Company | Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition	15,822
Non-accretable difference (expected losses and foregone interest)	(4,646)
Cash flows expected to be collected at acquisition	11,176
Accretable yield	0
Basis in acquired loans at acquisition	$ 11,176
Reliance Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition		$ 1,138,142
Non-accretable difference (expected losses and foregone interest)		0
Cash flows expected to be collected at acquisition		1,138,142
Accretable yield		(41,447)
Basis in acquired loans at acquisition		1,096,695
Reliance Bancshares, Inc. | Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition		385
Non-accretable difference (expected losses and foregone interest)		(210)
Cash flows expected to be collected at acquisition		175
Accretable yield
Basis in acquired loans at acquisition		$ 175
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