BUSINESS COMBINATIONS - Pro Forma Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Combinations [Abstract]
Net interest income and noninterest income	$ 1,084,253	$ 828,612	$ 803,281
Net income	263,089	228,798	196,352
Net income available to common shareholders	$ 263,089	$ 228,798	$ 196,352
Income per common share available to common shareholders – basic (in dollars per share)	$ 3.80	$ 3.29	$ 2.82
Income per common share available to common shareholders – diluted (in dollars per share)	$ 3.79	$ 3.29	$ 2.82
Average number of shares outstanding, basic (in shares)	69,256	69,462	69,642
Average number of shares outstanding, diluted (in shares)	69,426	69,614	69,748