Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Impact of ASU 2016-13 adoption	Preferred Stock	Common Stock	Surplus	Accumulated Other Comprehensive Income (Loss)	Undivided Profits	Undivided Profits Impact of ASU 2016-13 adoption	Reliance Bancshares, Inc.	Reliance Bancshares, Inc. Preferred Stock	Reliance Bancshares, Inc. Common Stock	Reliance Bancshares, Inc. Surplus	Landrum Company	Landrum Company Preferred Stock	Landrum Company Common Stock	Landrum Company Surplus
Balance at Dec. 31, 2017	$ 2,084,564		$ 0	$ 920	$ 1,586,034	$ (17,264)	$ 514,874
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	205,603					(10,110)	215,713
Stock issued for employee stock purchase plan	1,026				1,026
Stock-based compensation plans, net	10,887			3	10,884
Dividends on common stock	(55,646)						(55,646)
Balance at Dec. 31, 2018	$ 2,246,434		0	923	1,597,944	(27,374)	674,941
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Comprehensive income	$ 286,432					48,265	238,167
Stock issued for employee stock purchase plan	1,312			1	1,311
Stock-based compensation plans, net	10,532			3	10,529
Stock issued for acquisition									$ 144,830	$ 42,000	$ 40	$ 102,790	$ 415,772	$ 767	$ 173	$ 414,832
Preferred stock retirement	(42,000)		(42,000)
Stock repurchases	(10,128)			(4)	(10,124)
Dividends on preferred stock	(339)						(339)
Dividends on common stock	(63,921)						(63,921)
Balance at Dec. 31, 2019	2,988,924	$ (128,101)	767	1,136	2,117,282	20,891	848,848	$ (128,101)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	293,739					38,835	254,904
Stock issued for employee stock purchase plan	956			1	955
Stock-based compensation plans, net	9,110			3	9,107
Stock repurchases	(113,327)			(59)	(113,268)
Dividends on preferred stock	(52)						(52)
Dividends on common stock	(74,593)						(74,593)
Balance at Dec. 31, 2020	$ 2,976,656		$ 767	$ 1,081	$ 2,014,076	$ 59,726	$ 901,006