Stock Options, Restricted Stock, and Dividend Reinvestment Plans - Summary of Restricted and Performance Stock Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shares/ Units:
Beginning balance (in shares)	4,709,987
Shares/units converted from IBKC (in shares)	2,663,116
Shares/units granted (in shares)	2,610,929
Shares/units vested (in shares)	(1,551,877)
Shares/units cancelled (in shares)	(161,939)
Ending balance (in shares)	8,270,216	4,709,987
Weighted average grant date fair value (per share)
Nonvested beginning balance (in dollars per share)	$ 16.25
Shares/units converted from IBKC (in dollars per share)	9.40
Shares/units granted (in dollars per share)	9.89	$ 16.25	$ 18.70
Shares/units vested (in dollars per share)	15.15
Shares/units canceled (in dollars per share)	12.55
Nonvested ending balance (in dollars per share)	$ 12.47	$ 16.25
Percent of performance achieved for nonvested performance units	100.00%