Goodwill and Other Intangible Assets - Changes in Goodwill and Other Intangibles (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period	$ 127,340
Amortization of intangibles	(13,495)	$ (11,805)	$ (11,009)
Total	127,340	127,340
Core deposit premiums, end of period	111,110	127,340
Core deposit intangible
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period	111,808	79,807
Acquisition of intangible asset	0	42,695
Disposition of intangible asset	(2,324)	0
Amortization of intangibles	(12,121)	(10,694)	(9,500)
Total	97,363	111,808	79,807
Core deposit premiums, end of period	97,363	111,808	79,807
Books of Business Intangible
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period	15,532	11,527
Acquisition of intangible asset	0	5,116
Disposition of intangible asset	(413)	0
Amortization of intangibles	(1,372)	(1,111)	(1,400)
Total	13,747	11,527	11,527
Core deposit premiums, end of period	13,747	15,532	$ 11,527
Landrum Company | Core deposit intangible
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period	24,300
Total	24,300	24,300
Core deposit premiums, end of period		24,300
Reliance Bancshares, Inc. | Core deposit intangible
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period	18,400
Total	18,400	18,400
Core deposit premiums, end of period		18,400
Wealth Management Operations | Landrum Company | Core deposit intangible
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period	5,100
Total	$ 5,100	5,100
Core deposit premiums, end of period		$ 5,100