Goodwill and Other Intangible Assets - Changes in Goodwill and Other Intangibles (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jun. 30, 2019
Finite-lived Intangible Assets [Roll Forward]
Core deposit premiums, beginning of period	$ 127,340
Amortization of intangibles	(3,413)	$ (2,641)
Total	121,673		$ 127,340
Core deposit premiums, end of period	121,673		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	(1,841)		0
Amortization of intangibles	(3,070)		(10,694)
Total	106,897	79,807	79,807
Core deposit premiums, end of period	106,897		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	(343)		(1,111)
Total	14,776	$ 11,527	15,532
Core deposit premiums, end of period	14,776		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]
Total				$ 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