Acquisitions - Textual (Details) $ in Thousands			12 Months Ended
	Oct. 19, 2017 USD ($) financial_center branch shares	May 15, 2017 USD ($) branch shares	Dec. 31, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 842,651	$ 845,687
OKSB Merger
Business Acquisition [Line Items]
Shares issued (in shares) | shares	14,488,604
Shares issued	$ 431,400
Payments to acquire businesses, gross	$ 94,900
Number of bank branches | branch	29
Total assets acquired	$ 2,690,915
Gross value of loans acquired	1,996,453		2,021,000
Total deposits acquired	1,966,355
Goodwill	$ 229,144
Revenue attributable to acquisition			29,200
Net income attributable to acquisition			10,500
First Texas Merger
Business Acquisition [Line Items]
Shares issued (in shares) | shares	12,999,840
Shares issued	$ 387,100
Payments to acquire businesses, gross	$ 70,000
Number of bank branches | financial_center	15
Total assets acquired	$ 2,427,226
Gross value of loans acquired	2,208,378		2,246,000
Total deposits acquired	1,881,658
Goodwill	$ 240,843
Revenue attributable to acquisition			27,600
Net income attributable to acquisition			5,700
OKSB and First Texas Acquisitions
Business Acquisition [Line Items]
Acquisition related costs			$ 9,400
Hardeman County Investment Company, Inc.
Business Acquisition [Line Items]
Shares issued (in shares) | shares		1,599,940
Shares issued		$ 42,600
Payments to acquire businesses, gross		$ 30,000
Number of bank branches | branch		10
Total assets acquired		$ 462,924
Gross value of loans acquired		251,649			$ 254,200
Total deposits acquired		388,976
Goodwill		$ 29,371