Acquisitions - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Oct. 19, 2017	May 15, 2017	Sep. 30, 2018	Dec. 31, 2017	Oct. 18, 2017	Jun. 30, 2017	May 14, 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				$ 79,517
Investment securities	484,173				485,468
Loans acquired	1,996,453			2,021,000	2,039,524
Allowance for loan losses	0				(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
Assets Acquired
Intangibles	42,124				1,933
OKSB Merger | Other intangibles
Assets Acquired
Intangibles	3,806				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
Intangibles	40,191
OKSB Merger | Fair Value Adjustments | Other intangibles
Assets Acquired
Intangibles	0
First Texas Merger
Assets Acquired
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
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 | Core deposit intangible
Assets Acquired
Intangibles	7,328				$ 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
Intangibles	$ 7,328
Hardeman County Investment Company, Inc.
Assets Acquired
Cash and due from banks		$ 8,001					$ 8,001
Interest bearing balances 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
Assets Acquired
Intangibles		7,840					0
Hardeman County Investment Company, Inc. | Other intangibles
Assets Acquired
Intangibles		830					$ 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
Intangibles		7,840
Hardeman County Investment Company, Inc. | Fair Value Adjustments | Other intangibles
Assets Acquired
Intangibles		$ 830