Acquisitions - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Oct. 19, 2017	May 15, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Assets Acquired
Goodwill			$ 845,687	$ 842,651
Liabilities Assumed
Goodwill			$ 845,687	842,651
OKSB Merger
Assets Acquired
Cash and due from banks	$ 79,517
Investment securities	484,173
Loans acquired	1,996,453			2,021,000
Allowance for loan losses	0
Foreclosed assets	5,157
Premises and equipment	26,667
Bank owned life insurance	28,704
Goodwill	229,144
Other assets	24,314
Total assets acquired	2,690,915
Liabilities Assumed
Non-interest bearing transaction accounts	485,971
Interest bearing transaction accounts and savings deposits	869,252
Time deposits	611,132
Total deposits	1,966,355
Securities sold under agreement to repurchase	11,256
Other borrowings	347,000
Subordinated debentures	46,393
Accrued interest and other liabilities	22,804
Total liabilities assumed	2,393,808
Total liabilities and equity assumed	2,393,808
Net assets acquired	297,107
Purchase price	526,251
Goodwill	229,144
OKSB Merger | Core deposit intangible
Assets Acquired
Intangible	42,124
OKSB Merger | Other intangibles
Assets Acquired
Intangible	3,806
OKSB Merger | Acquisition-related Costs
Assets Acquired
Cash and due from banks	79,517
Investment securities	485,468
Loans acquired	2,039,524
Allowance for loan losses	(26,957)
Foreclosed assets	6,284
Premises and equipment	21,210
Bank owned life insurance	28,704
Goodwill	13,545
Other assets	33,455
Total assets acquired	2,686,489
Liabilities Assumed
Non-interest bearing transaction accounts	485,971
Interest bearing transaction accounts and savings deposits	869,252
Time deposits	613,345
Total deposits	1,968,568
Securities sold under agreement to repurchase	11,256
Other borrowings	347,000
Subordinated debentures	46,393
Accrued interest and other liabilities	17,440
Total liabilities assumed	2,390,657
Total equity assumed	295,832
Total liabilities and equity assumed	2,686,489
Goodwill	13,545
OKSB Merger | Acquisition-related Costs | Core deposit intangible
Assets Acquired
Intangible	1,933
OKSB Merger | Acquisition-related Costs | Other intangibles
Assets Acquired
Intangible	3,806
OKSB Merger | Fair Value Adjustments
Assets Acquired
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
Assets Acquired
Intangible	40,191
OKSB Merger | Fair Value Adjustments | Other intangibles
Assets Acquired
Intangible	0
Hardeman County Investment Company, Inc.
Assets Acquired
Cash and due from banks		$ 8,001
Interest bearing balances due from banks - time		1,984
Investment securities		170,369
Loans acquired		251,649			$ 254,200
Foreclosed assets		631
Premises and equipment		11,163
Bank owned life insurance		7,819
Goodwill		29,371
Other assets		2,638
Total assets acquired		462,924
Liabilities Assumed
Non-interest bearing transaction accounts		76,555
Interest bearing transaction accounts and savings deposits		214,872
Time deposits		97,549
Total deposits		388,976
Securities sold under agreement to repurchase		17,163
Other borrowings		3,000
Subordinated debentures		6,702
Accrued interest and other liabilities		3,815
Total liabilities assumed		419,656
Total liabilities and equity assumed		419,656
Net assets acquired		43,268
Purchase price		72,639
Goodwill		29,371
Hardeman County Investment Company, Inc. | Core deposit intangible
Assets Acquired
Intangible		7,840
Hardeman County Investment Company, Inc. | Other intangibles
Assets Acquired
Intangible		830
Hardeman County Investment Company, Inc. | Acquisition-related Costs
Assets Acquired
Cash and due from banks		8,001
Interest bearing balances due from banks - time		1,984
Investment securities		170,654
Loans acquired		257,641
Allowance for loan losses		(2,382)
Foreclosed assets		1,083
Premises and equipment		9,905
Bank owned life insurance		7,819
Goodwill		11,485
Other assets		2,639
Total assets acquired		468,829
Liabilities Assumed
Non-interest bearing transaction accounts		76,555
Interest bearing transaction accounts and savings deposits		214,872
Time deposits		97,917
Total deposits		389,344
Securities sold under agreement to repurchase		17,163
Other borrowings		3,000
Subordinated debentures		6,702
Accrued interest and other liabilities		1,891
Total liabilities assumed		418,100
Total equity assumed		50,729
Total liabilities and equity assumed		468,829
Goodwill		11,485
Hardeman County Investment Company, Inc. | Acquisition-related Costs | Core deposit intangible
Assets Acquired
Intangible		0
Hardeman County Investment Company, Inc. | Acquisition-related Costs | Other intangibles
Assets Acquired
Intangible		0
Hardeman County Investment Company, Inc. | Fair Value Adjustments
Assets Acquired
Cash and due from banks		0
Interest bearing balances 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
Assets Acquired
Intangible		7,840
Hardeman County Investment Company, Inc. | Fair Value Adjustments | Other intangibles
Assets Acquired
Intangible		$ 830
First Texas Merger
Assets Acquired
Cash and due from banks	59,277
Investment securities	80,518
Loans acquired	2,208,378			$ 2,246,000
Allowance for loan losses	(200)
Premises and equipment	34,987
Bank owned life insurance	7,190
Goodwill	240,843
Other assets	29,748
Total assets acquired	2,427,226
Liabilities Assumed
Non-interest bearing transaction accounts	74,410
Interest bearing transaction accounts and savings deposits	1,683,298
Time deposits	123,950
Total deposits	1,881,658
Securities sold under agreement to repurchase	50,000
Other borrowings	235,000
Subordinated debentures	30,912
Accrued interest and other liabilities	13,396
Total liabilities assumed	2,210,966
Total liabilities and equity assumed	2,210,966
Net assets acquired	216,260
Purchase price	457,103
Goodwill	240,843
First Texas Merger | Core deposit intangible
Assets Acquired
Intangible	7,328
First Texas Merger | Acquisition-related Costs
Assets Acquired
Cash and due from banks	59,277
Investment securities	81,114
Loans acquired	2,246,212
Allowance for loan losses	(20,864)
Premises and equipment	24,864
Bank owned life insurance	7,190
Goodwill	37,227
Other assets	18,263
Total assets acquired	2,453,283
Liabilities Assumed
Non-interest bearing transaction accounts	74,410
Interest bearing transaction accounts and savings deposits	1,683,298
Time deposits	124,233
Total deposits	1,881,941
Securities sold under agreement to repurchase	50,000
Other borrowings	235,000
Subordinated debentures	30,323
Accrued interest and other liabilities	11,727
Total liabilities assumed	2,208,991
Total equity assumed	244,292
Total liabilities and equity assumed	2,453,283
Goodwill	37,227
First Texas Merger | Acquisition-related Costs | Core deposit intangible
Assets Acquired
Intangible	0
First Texas Merger | Fair Value Adjustments
Assets Acquired
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)
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)
First Texas Merger | Fair Value Adjustments | Core deposit intangible
Assets Acquired
Intangible	$ 7,328