Acquisitions (Pro forma information) (Details) - First Bank Lubbock Bancshares, Inc. - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Net interest income	$ 317,846	$ 262,367
Net income available to common stockholders	$ 69,075	$ 76,698
Basic earnings per share (in dollars per share)	$ 2.02	$ 2.48
Diluted earnings per share (in dollars per share)	$ 2.01	$ 2.46