Goodwill and Other Intangible Assets - Changes in Goodwill and Other Intangibles (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2019
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period	$ 91,334
Amortization of intangibles	(11,805)	$ (11,009)	$ (7,668)
Total	127,340	91,334
Core deposit premiums, end of period	127,340	91,334
Core deposit intangible
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period	79,807	89,325
Acquisition of intangible asset	42,695	0
Amortization of intangibles	(10,694)	(9,518)	(6,000)
Total	79,807	79,807	89,325
Core deposit premiums, end of period	111,808	79,807	89,325
Books of Business Intangible
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period	11,527	16,746
Acquisition of intangible asset	5,116	0
Disposition of intangible asset	0	(3,849)
Amortization of intangibles	(1,111)	(1,370)	(1,600)
Total	11,527	11,527	16,746
Core deposit premiums, end of period	15,532	$ 11,527	$ 16,746
Landrum Company | Core deposit intangible
Finite-lived Intangible Assets [Roll Forward]
Total	24,300
Core deposit premiums, end of period	24,300
Reliance Bancshares, Inc. | Core deposit intangible
Finite-lived Intangible Assets [Roll Forward]
Total				$ 18,400
Wealth Management Operations | Landrum Company | Core deposit intangible
Finite-lived Intangible Assets [Roll Forward]
Total	5,100
Core deposit premiums, end of period	$ 5,100