Goodwill and Other Intangible Assets - Changes in Goodwill and Other Intangibles (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period			$ 127,340
Amortization of intangibles	$ (3,362)	$ (2,947)	(10,144)	$ (8,535)
Total	114,460		114,460		$ 127,340
Core deposit premiums, end of period	114,460		114,460		127,340
Core deposit intangible
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period			111,808	79,807	79,807
Acquisition of intangible asset			0		42,695
Disposition of intangible asset			(2,324)		0
Amortization of intangibles			(9,114)		(10,694)
Total	100,370		100,370	79,807	79,807
Core deposit premiums, end of period	100,370		100,370		111,808
Books of Business Intangible
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period			15,532	11,527	11,527
Acquisition of intangible asset			0		5,116
Disposition of intangible asset			(413)		0
Amortization of intangibles			(1,029)		(1,111)
Total	14,090		14,090	$ 11,527	11,527
Core deposit premiums, end of period	$ 14,090		14,090		15,532
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