Stock Options, Restricted Stock, and Dividend Reinvestment Plans - Summary of Stock Option Activity (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) $ / shares shares
Options Outstanding
Beginning of period, outstanding (in shares) | shares	4,931,781
Converted IBKC (in shares) | shares	3,597,856
Options granted (in shares) | shares	584,881
Options exercised (in shares) | shares	(597,686)
Options expired/cancelled (in shares) | shares	(767,750)
End of period, outstanding (in shares) | shares	7,749,082
Options exercisable (in shares) | shares	5,766,528
Options expected to vest (in shares) | shares	1,982,554
Weighted Average Exercise Price (per share)
Beginning of period, weighted average exercise price (in dollars per share) | $ / shares	$ 15.61
Converted IBKC (in dollars per share) | $ / shares	14.39
Options granted, weighted average exercise price (in dollars per share) | $ / shares	15.90
Options exercised, weighted average exercise price (in dollars per share) | $ / shares	11.55
Options expired/cancelled, weighted average exercise price (in dollars per share) | $ / shares	17.44
End of period, weighted average exercise price (in dollars per share) | $ / shares	15.20
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	15.02
Options expected to vest, weighted average exercise price (in dollars per share) | $ / shares	$ 15.70
Outstanding, weighted average contractual term	3 years 10 months 6 days
Outstanding, aggregate intrinsic value | $	$ 5,000
Options exercisable, weighted average contractual term	3 years 1 month 28 days
Options exercisable, aggregate intrinsic value | $	$ 4,000
Options expected to vest, weighted average remaining contractual term	5 years 10 months 9 days
Options expected to vest, aggregate intrinsic value | $	$ 0