Acquisitions (Pro Forma Information (unaudited)) (Details) - First Bank Lubbock Bancshares, Inc. - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Net interest income	$ 428,129	$ 364,993
Net income available to common stockholders	$ 101,216	$ 96,346
Basic earnings per share (in dollars per share)	$ 2.95	$ 3.06
Diluted earnings per share (in dollars per share)	$ 2.93	$ 3.03