Acquisitions (Details) $ / shares in Units, $ in Thousands					12 Months Ended
	Oct. 31, 2019 USD ($) $ / shares shares	Apr. 12, 2019 USD ($) shares	Oct. 19, 2017 USD ($) branch shares	May 15, 2017 USD ($) branch shares	Dec. 31, 2019 USD ($) financial_center $ / shares shares	Dec. 31, 2017 USD ($)	Oct. 30, 2019 USD ($) financial_center	Oct. 29, 2019 $ / shares shares	Jun. 30, 2019 USD ($)	Apr. 11, 2019 USD ($) financial_center	Dec. 31, 2018 USD ($)	Oct. 18, 2017 USD ($)	Jun. 30, 2017 USD ($)	May 14, 2017 USD ($)	Feb. 27, 2009 $ / shares shares
Business Acquisition [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares					$ 0.01										$ 0.01
Number of financial centers | financial_center					251
Goodwill					$ 1,055,520						$ 845,687
Preferred stock, shares authorized (in shares) | shares					40,040,000										40,040,000
Preferred stock, shares issued (in shares) | shares					767
Landrum Company
Business Acquisition [Line Items]
Equity interest issued or issuable (in shares) | shares	17,349,722
Number of financial centers | financial_center							39
Assets acquired	$ 3,407,320						$ 3,381,889
Loans acquired	2,005,936				$ 2,011,000		2,049,137
Deposits acquired	3,049,600						3,049,301
Goodwill	$ 131,340				131,300		$ 407
Stock issued for acquisition					415,772
Reliance Bancshares, Inc.
Business Acquisition [Line Items]
Equity interest issued or issuable (in shares) | shares		3,999,623
Number of financial centers | financial_center										22
Assets acquired		$ 1,534,312								$ 1,549,954
Loans acquired		1,096,870							$ 1,127,000	1,138,527
Deposits acquired		1,225,267								$ 1,227,058
Goodwill		78,493			78,500
Payments to acquire businesses, gross		62,700
Stock issued for acquisition		$ 42,000			$ 144,830
OKSB Merger
Business Acquisition [Line Items]
Equity interest issued or issuable (in shares) | shares			14,488,604
Assets acquired			$ 2,690,915									$ 2,686,489
Loans acquired			1,996,453								2,021,000	2,039,524
Deposits acquired			1,966,355									1,968,568
Goodwill			229,144									13,545
Payments to acquire businesses, gross			94,900
Stock issued for acquisition						$ 431,398
Equity interest issued or issuable, value assigned			$ 431,400
Number of bank branches | branch			29
First Texas Merger
Business Acquisition [Line Items]
Equity interest issued or issuable (in shares) | shares			12,999,840
Assets acquired			$ 2,427,226									2,453,283
Loans acquired			2,208,378								$ 2,246,000	2,246,212
Deposits acquired			1,881,658									1,881,941
Goodwill			240,843									$ 37,227
Payments to acquire businesses, gross			70,000
Stock issued for acquisition						387,070
Equity interest issued or issuable, value assigned			$ 387,100
Number of bank branches | branch			15
Hardeman County Investment Company, Inc.
Business Acquisition [Line Items]
Equity interest issued or issuable (in shares) | shares				1,599,940
Assets acquired				$ 462,924										$ 468,829
Loans acquired				251,649									$ 254,200	257,641
Deposits acquired				388,976										389,344
Goodwill				29,371										$ 11,485
Payments to acquire businesses, gross				30,000
Stock issued for acquisition						$ 42,638
Equity interest issued or issuable, value assigned				$ 42,600
Number of bank branches | branch				10
Common Class A | Landrum Company
Business Acquisition [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.01
Preferred Class B | Landrum Company
Business Acquisition [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.01
Series E Preferred Stock | Landrum Company
Business Acquisition [Line Items]
Equity interest issued or issuable, conversion ratio (in shares)	17,350,000
Series D Preferred Stock | Landrum Company
Business Acquisition [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.01
Equity interest issued or issuable, conversion ratio (in shares)	1
Series D Preferred Stock | Reliance Bancshares, Inc.
Business Acquisition [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares								$ 0.01
Preferred Stock | Landrum Company
Business Acquisition [Line Items]
Equity interest issued or issuable (in shares) | shares	767
Series A or B Preferred Stock | Reliance Bancshares, Inc.
Business Acquisition [Line Items]
Equity interest issued or issuable, conversion ratio (in shares)		1
Series C Preferred Stock | Reliance Bancshares, Inc.
Business Acquisition [Line Items]
Equity interest issued or issuable, conversion ratio (in shares)		1
Preferred stock, shares authorized (in shares) | shares								140
Preferred stock, shares issued (in shares) | shares								0