Acquisitions (Details)				12 Months Ended
	Oct. 19, 2017 USD ($) branch shares	May 15, 2017 USD ($) branch shares	Sep. 09, 2016 USD ($) branch shares	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Oct. 18, 2017 USD ($)	Jun. 30, 2017 USD ($)	May 14, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 08, 2016 USD ($)
Business Acquisition [Line Items]
Goodwill				$ 842,651,000	$ 845,687,000
OKSB Merger
Business Acquisition [Line Items]
Equity interest issued or issuable (in shares) | shares	14,488,604
Equity interest issued or issuable, value assigned	$ 431,400,000
Payments to acquire businesses, gross	$ 94,900,000
Number of bank branches | branch	29
Assets acquired	$ 2,690,915,000					$ 2,686,489,000
Loans acquired	1,996,453,000			2,021,000,000		2,039,524,000
Deposits acquired	1,966,355,000					1,968,568,000
Goodwill	$ 229,144,000					13,545,000
Revenue of acquiree since acquisition date, actual				29,200,000
Earnings or loss of acquiree since acquisition date, actual				10,500,000
First Texas Merger
Business Acquisition [Line Items]
Equity interest issued or issuable (in shares) | shares	12,999,840
Equity interest issued or issuable, value assigned	$ 387,100,000
Payments to acquire businesses, gross	$ 70,000,000
Number of bank branches | branch	15
Assets acquired	$ 2,427,226,000					2,453,283,000
Loans acquired	2,208,378,000			2,246,000,000		2,246,212,000
Deposits acquired	1,881,658,000					1,881,941,000
Goodwill	$ 240,843,000					$ 37,227,000
Revenue of acquiree since acquisition date, actual				27,600,000
Earnings or loss of acquiree since acquisition date, actual				5,700,000
OKSB and First Texas Acquisitions
Business Acquisition [Line Items]
Acquisition related costs				$ 9,400,000
Hardeman County Investment Company, Inc.
Business Acquisition [Line Items]
Equity interest issued or issuable (in shares) | shares		1,599,940
Equity interest issued or issuable, value assigned		$ 42,600,000
Payments to acquire businesses, gross		$ 30,000,000
Number of bank branches | branch		10
Assets acquired		$ 462,924,000						$ 468,829,000
Loans acquired		251,649,000					$ 254,200,000	257,641,000
Deposits acquired		388,976,000						389,344,000
Goodwill		$ 29,371,000						$ 11,485,000
Citizens National Bank
Business Acquisition [Line Items]
Equity interest issued or issuable (in shares) | shares			1,671,482
Equity interest issued or issuable, value assigned			$ 41,300,000
Payments to acquire businesses, gross			$ 35,000,000
Number of bank branches | branch			9
Assets acquired			$ 585,149,000								$ 585,204,000
Loans acquired			340,850,000							$ 348,800,000	350,361,000
Deposits acquired			509,857,000								$ 509,857,000
Goodwill			$ 23,376,000						$ 21,500,000