Business Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Noninterest income:
Deposit transactions and cash management	$ 46,000	$ 43,000	$ 133,000	$ 129,000
Fixed income	28,000	46,000	97,000	170,000
Brokerage, management fees and commissions	21,000	23,000	66,000	71,000
Card and digital banking fees	20,000	21,000	61,000	64,000
Other service charges and fees	14,000	13,000	41,000	41,000
Trust services and investment management	12,000	11,000	35,000	36,000
Mortgage banking and title income	7,000	9,000	18,000	65,000
Deferred compensation income	0	(3,000)	11,000	(24,000)
Gain on merger termination	0	0	225,000	0
Securities gains (losses), net	0	12,000	1,000	18,000
Other income	25,000	38,000	57,000	72,000
Total noninterest income	173,000	213,000	745,000	642,000
Underwriting, portfolio advisory, and other noninterest income
Noninterest income:
Revenue from contract with customer	13,000	13,000	32,000	33,000
Regional Banking
Noninterest income:
Deposit transactions and cash management	41,000	39,000	120,000	115,000
Fixed income	0	0	0	0
Brokerage, management fees and commissions	21,000	23,000	66,000	71,000
Card and digital banking fees	19,000	19,000	55,000	57,000
Other service charges and fees	7,000	8,000	23,000	24,000
Trust services and investment management	12,000	11,000	35,000	36,000
Mortgage banking and title income	0	0	0	0
Deferred compensation income		0	0	0
Gain on merger termination			0
Securities gains (losses), net		0	0	0
Other income	9,000	10,000	26,000	34,000
Total noninterest income	109,000	110,000	325,000	337,000
Specialty Banking
Noninterest income:
Deposit transactions and cash management	3,000	2,000	7,000	7,000
Fixed income	28,000	46,000	98,000	170,000
Brokerage, management fees and commissions	0	0	0	0
Card and digital banking fees	0	0	1,000	2,000
Other service charges and fees	7,000	5,000	18,000	17,000
Trust services and investment management	0	0	0	0
Mortgage banking and title income	7,000	9,000	18,000	65,000
Deferred compensation income		0	0	0
Gain on merger termination			0
Securities gains (losses), net		0	0	0
Other income	1,000	2,000	5,000	4,000
Total noninterest income	46,000	64,000	147,000	265,000
Corporate
Noninterest income:
Deposit transactions and cash management	2,000	2,000	6,000	7,000
Fixed income	0	0	(1,000)	0
Brokerage, management fees and commissions	0	0	0	0
Card and digital banking fees	1,000	2,000	5,000	5,000
Other service charges and fees	0	0	0	0
Trust services and investment management	0	0	0	0
Mortgage banking and title income	0	0	0	0
Deferred compensation income		(3,000)	11,000	(24,000)
Gain on merger termination			225,000
Securities gains (losses), net		12,000	1,000	18,000
Other income	15,000	26,000	26,000	34,000
Total noninterest income	$ 18,000	$ 39,000	$ 273,000	$ 40,000