Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
ASSETS
Cash and cash equivalents	$ 3,472,152	$ 996,623
Loans	12,900,897	14,425,704
Other assets	289,332	241,578
Total assets	22,359,752	21,259,143		$ 15,000,000
LIABILITIES
Long-term debt	1,724,941
Total liabilities	19,383,096	18,270,219
Stockholders’ equity:
Preferred stock	767	767
Common stock	1,081	1,136
Surplus	2,014,076	2,117,282
Accumulated other comprehensive gain (loss):	901,006	848,848
Total stockholders’ equity	2,976,656	2,988,924	$ 2,246,434	$ 2,084,564
Total liabilities and stockholders’ equity	22,359,752	21,259,143
Parent Company
ASSETS
Cash and cash equivalents	172,083	104,068
Investment securities	1,648	743
Investments in wholly-owned subsidiaries	3,165,791	3,269,224
Loans	471	657
Intangible assets, net	133	133
Premises and equipment	25,606	27,351
Other assets	56,257	31,738
Total assets	3,421,989	3,433,914
LIABILITIES
Long-term debt	406,859	413,760
Other liabilities	38,474	31,230
Total liabilities	445,333	444,990
Stockholders’ equity:
Preferred stock	767	767
Common stock	1,081	1,136
Surplus	2,014,076	2,117,282
Accumulated other comprehensive gain (loss):	901,006	848,848
Unrealized appreciation on available-for-sale securities, net of income taxes of $21,132 and $7,392 at December 31, 2020 and 2019 respectively	59,726	20,891
Total stockholders’ equity	2,976,656	2,988,924
Total liabilities and stockholders’ equity	$ 3,421,989	$ 3,433,914