Acquisitions - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Oct. 31, 2019	Apr. 12, 2019	Oct. 19, 2017	May 15, 2017	Dec. 31, 2019	Oct. 30, 2019	Jun. 30, 2019	Apr. 11, 2019	Dec. 31, 2018	Oct. 18, 2017	Jun. 30, 2017	May 14, 2017
Business Acquisition [Line Items]
Goodwill					$ 1,055,520				$ 845,687
Liabilities Assumed
Goodwill					1,055,520				845,687
Landrum Company
Business Acquisition [Line Items]
Cash and due from banks	$ 215,285					$ 215,285
Due from banks - time	248					248
Investment securities	1,025,855					1,021,755
Loans acquired	2,005,936				2,011,000	2,049,137
Allowance for loan losses						(22,736)
Foreclosed assets	190					373
Premises and equipment	84,466					63,878
Bank owned life insurance	19,206					19,206
Goodwill	131,340				131,300	407
Other assets	26,673					33,924
Total assets acquired	3,407,320					3,381,889
Liabilities Assumed
Non-interest bearing transaction accounts	716,675					716,675
Interest bearing transaction accounts and savings deposits	1,465,429					1,465,429
Time deposits	867,496					867,197
Total deposits	3,049,600					3,049,301
Other borrowings	10,055					10,055
Subordinated debentures	33,917					34,794
Accrued interest and other liabilities	29,309					31,057
Total liabilities assumed	3,122,881					3,125,207
Total equity assumed						256,682
Total liabilities and equity assumed	3,122,881					3,381,889
Net assets acquired	284,439
Purchase price	415,779
Goodwill	131,340				131,300	407
Landrum Company | Core deposit intangible
Business Acquisition [Line Items]
Core deposit intangible	24,345					0
Landrum Company | Other intangibles
Business Acquisition [Line Items]
Core deposit intangible	5,116					$ 412
Landrum Company | Fair Value Adjustments
Business Acquisition [Line Items]
Cash and due from banks	0
Due from banks - time	0
Investment securities	4,100
Loans acquired	(43,201)
Allowance for loan losses	22,736
Foreclosed assets	(183)
Premises and equipment	20,588
Bank owned life insurance	0
Goodwill	(407)
Other assets	(7,251)
Total assets acquired	25,431
Liabilities Assumed
Non-interest bearing transaction accounts	0
Interest bearing transaction accounts and savings deposits	0
Time deposits	299
Total deposits	299
Other borrowings	0
Subordinated debentures	(877)
Accrued interest and other liabilities	(1,748)
Total liabilities assumed	(2,326)
Total equity assumed	(256,682)
Total liabilities and equity assumed	(259,008)
Goodwill	(407)
Landrum Company | Fair Value Adjustments | Core deposit intangible
Business Acquisition [Line Items]
Core deposit intangible	24,345
Landrum Company | Fair Value Adjustments | Other intangibles
Business Acquisition [Line Items]
Core deposit intangible	$ 4,704
Reliance Bancshares, Inc.
Business Acquisition [Line Items]
Cash and due from banks		$ 25,693						$ 25,693
Due from banks - time		502						502
Investment securities		286,110						287,983
Loans acquired		1,096,870					$ 1,127,000	1,138,527
Allowance for loan losses		0						(10,808)
Foreclosed assets		5,912						11,092
Premises and equipment		29,451						32,452
Bank owned life insurance		39,348						39,348
Goodwill		78,493			78,500
Core deposit intangible		18,350						0
Other assets		32,076						25,165
Total assets acquired		1,534,312						1,549,954
Liabilities Assumed
Non-interest bearing transaction accounts		108,812						108,845
Interest bearing transaction accounts and savings deposits		639,798						639,798
Time deposits		476,657						478,415
Total deposits		1,225,267						1,227,058
Securities sold under agreement to repurchase		14,146						14,146
Other borrowings		157,400						162,900
Accrued interest and other liabilities		8,453						8,185
Total liabilities assumed		1,405,266						1,412,289
Total equity assumed								137,665
Total liabilities and equity assumed		1,405,266						$ 1,549,954
Net assets acquired		129,046
Purchase price		207,539
Goodwill		78,493			$ 78,500
Reliance Bancshares, Inc. | Fair Value Adjustments
Business Acquisition [Line Items]
Cash and due from banks		0
Due from banks - time		0
Investment securities		(1,873)
Loans acquired		(41,657)
Allowance for loan losses		10,808
Foreclosed assets		(5,180)
Premises and equipment		(3,001)
Bank owned life insurance		0
Core deposit intangible		18,350
Other assets		6,911
Total assets acquired		(15,642)
Liabilities Assumed
Non-interest bearing transaction accounts		(33)
Interest bearing transaction accounts and savings deposits		0
Time deposits		(1,758)
Total deposits		(1,791)
Securities sold under agreement to repurchase		0
Other borrowings		(5,500)
Accrued interest and other liabilities		268
Total liabilities assumed		(7,023)
Total equity assumed		(137,665)
Total liabilities and equity assumed		$ (144,688)
OKSB Merger
Business Acquisition [Line Items]
Cash and due from banks			$ 79,517							$ 79,517
Investment securities			484,173							485,468
Loans acquired			1,996,453						2,021,000	2,039,524
Allowance for loan losses										(26,957)
Foreclosed assets			5,157							6,284
Premises and equipment			26,667							21,210
Bank owned life insurance			28,704							28,704
Goodwill			229,144							13,545
Other assets			24,314							33,455
Total assets acquired			2,690,915							2,686,489
Liabilities Assumed
Non-interest bearing transaction accounts			485,971							485,971
Interest bearing transaction accounts and savings deposits			869,252							869,252
Time deposits			611,132							613,345
Total deposits			1,966,355							1,968,568
Securities sold under agreement to repurchase			11,256							11,256
Other borrowings			347,000							347,000
Subordinated debentures			46,393							46,393
Accrued interest and other liabilities			22,804							17,440
Total liabilities assumed			2,393,808							2,390,657
Total equity assumed										295,832
Total liabilities and equity assumed			2,393,808							2,686,489
Net assets acquired			297,107
Purchase price			526,251
Goodwill			229,144							13,545
OKSB Merger | Core deposit intangible
Business Acquisition [Line Items]
Core deposit intangible			42,124							1,933
OKSB Merger | Other intangibles
Business Acquisition [Line Items]
Core deposit intangible			3,806							3,806
OKSB Merger | Fair Value Adjustments
Business Acquisition [Line Items]
Cash and due from banks			0
Investment securities			(1,295)
Loans acquired			(43,071)
Allowance for loan losses			26,957
Foreclosed assets			(1,127)
Premises and equipment			5,457
Bank owned life insurance			0
Goodwill			(13,545)
Other assets			(9,141)
Total assets acquired			4,426
Liabilities Assumed
Non-interest bearing transaction accounts			0
Interest bearing transaction accounts and savings deposits			0
Time deposits			(2,213)
Total deposits			(2,213)
Securities sold under agreement to repurchase			0
Other borrowings			0
Subordinated debentures			0
Accrued interest and other liabilities			5,364
Total liabilities assumed			3,151
Total equity assumed			(295,832)
Total liabilities and equity assumed			(292,681)
Goodwill			(13,545)
OKSB Merger | Fair Value Adjustments | Core deposit intangible
Business Acquisition [Line Items]
Core deposit intangible			40,191
OKSB Merger | Fair Value Adjustments | Other intangibles
Business Acquisition [Line Items]
Core deposit intangible			0
First Texas Merger
Business Acquisition [Line Items]
Cash and due from banks			59,277							59,277
Investment securities			80,518							81,114
Loans acquired			2,208,378						$ 2,246,000	2,246,212
Allowance for loan losses			(200)							(20,864)
Premises and equipment			34,987							24,864
Bank owned life insurance			7,190							7,190
Goodwill			240,843							37,227
Core deposit intangible			7,328							0
Other assets			29,748							18,263
Total assets acquired			2,427,226							2,453,283
Liabilities Assumed
Non-interest bearing transaction accounts			74,410							74,410
Interest bearing transaction accounts and savings deposits			1,683,298							1,683,298
Time deposits			123,950							124,233
Total deposits			1,881,658							1,881,941
Securities sold under agreement to repurchase			50,000							50,000
Other borrowings			235,000							235,000
Subordinated debentures			30,912							30,323
Accrued interest and other liabilities			13,396							11,727
Total liabilities assumed			2,210,966							2,208,991
Total equity assumed										244,292
Total liabilities and equity assumed			2,210,966							2,453,283
Net assets acquired			216,260
Purchase price			457,103
Goodwill			240,843							$ 37,227
First Texas Merger | Fair Value Adjustments
Business Acquisition [Line Items]
Cash and due from banks			0
Investment securities			(596)
Loans acquired			(37,834)
Allowance for loan losses			20,664
Premises and equipment			10,123
Bank owned life insurance			0
Goodwill			(37,227)
Core deposit intangible			7,328
Other assets			11,485
Total assets acquired			(26,057)
Liabilities Assumed
Non-interest bearing transaction accounts			0
Interest bearing transaction accounts and savings deposits			0
Time deposits			(283)
Total deposits			(283)
Securities sold under agreement to repurchase			0
Other borrowings			0
Subordinated debentures			589
Accrued interest and other liabilities			1,669
Total liabilities assumed			1,975
Total equity assumed			(244,292)
Total liabilities and equity assumed			(242,317)
Goodwill			$ (37,227)
Hardeman County Investment Company, Inc.
Business Acquisition [Line Items]
Cash and due from banks				$ 8,001								$ 8,001
Due from banks - time				1,984								1,984
Investment securities				170,369								170,654
Loans acquired				251,649							$ 254,200	257,641
Allowance for loan losses												(2,382)
Foreclosed assets				631								1,083
Premises and equipment				11,163								9,905
Bank owned life insurance				7,819								7,819
Goodwill				29,371								11,485
Other assets				2,638								2,639
Total assets acquired				462,924								468,829
Liabilities Assumed
Non-interest bearing transaction accounts				76,555								76,555
Interest bearing transaction accounts and savings deposits				214,872								214,872
Time deposits				97,549								97,917
Total deposits				388,976								389,344
Securities sold under agreement to repurchase				17,163								17,163
Other borrowings				3,000								3,000
Subordinated debentures				6,702								6,702
Accrued interest and other liabilities				3,815								1,891
Total liabilities assumed				419,656								418,100
Total equity assumed												50,729
Total liabilities and equity assumed				419,656								468,829
Net assets acquired				43,268
Purchase price				72,639
Goodwill				29,371								11,485
Hardeman County Investment Company, Inc. | Core deposit intangible
Business Acquisition [Line Items]
Core deposit intangible				7,840								0
Hardeman County Investment Company, Inc. | Other intangibles
Business Acquisition [Line Items]
Core deposit intangible				830								$ 0
Hardeman County Investment Company, Inc. | Fair Value Adjustments
Business Acquisition [Line Items]
Cash and due from banks				0
Due from banks - time				0
Investment securities				(285)
Loans acquired				(5,992)
Allowance for loan losses				2,382
Foreclosed assets				(452)
Premises and equipment				1,258
Bank owned life insurance				0
Goodwill				(11,485)
Other assets				(1)
Total assets acquired				(5,905)
Liabilities Assumed
Non-interest bearing transaction accounts				0
Interest bearing transaction accounts and savings deposits				0
Time deposits				(368)
Total deposits				(368)
Securities sold under agreement to repurchase				0
Other borrowings				0
Subordinated debentures				0
Accrued interest and other liabilities				1,924
Total liabilities assumed				1,556
Total equity assumed				(50,729)
Total liabilities and equity assumed				(49,173)
Goodwill				(11,485)
Hardeman County Investment Company, Inc. | Fair Value Adjustments | Core deposit intangible
Business Acquisition [Line Items]
Core deposit intangible				7,840
Hardeman County Investment Company, Inc. | Fair Value Adjustments | Other intangibles
Business Acquisition [Line Items]
Core deposit intangible				$ 830