Business Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Noninterest income:
Fixed income	$ 96,000	$ 111,000	$ 324,000	$ 319,000
Deposit transactions and cash management	44,000	42,000	130,000	103,000
Mortgage banking and title income	34,000	66,000	126,000	72,000
Brokerage, management fees and commissions	24,000	18,000	65,000	47,000
Bankcard income	15,000	10,000	40,000	24,000
Trust services and investment management	13,000	12,000	39,000	27,000
Securities gains (losses), net	1,000	(1,000)	12,000	(2,000)
Loss on debt extinguishment	(23,000)	0	(23,000)	0
Purchase accounting gain	0	532,000	(1,000)	532,000
Other income	43,000	33,000	117,000	82,000
Total noninterest income	247,000	823,000	829,000	1,204,000
Regional Banking
Noninterest income:
Fixed income	0	0	1,000	1,000
Deposit transactions and cash management	39,000	38,000	117,000	91,000
Mortgage banking and title income	0	0	0	0
Brokerage, management fees and commissions	24,000	18,000	65,000	47,000
Bankcard income	14,000	10,000	37,000	23,000
Trust services and investment management	13,000	12,000	39,000	27,000
Securities gains (losses), net	0	0	0	0
Loss on debt extinguishment	0		0
Purchase accounting gain		0	0	0
Other income	22,000	19,000	61,000	50,000
Total noninterest income	112,000	97,000	320,000	239,000
Specialty Banking
Noninterest income:
Fixed income	96,000	111,000	323,000	318,000
Deposit transactions and cash management	3,000	2,000	8,000	8,000
Mortgage banking and title income	33,000	66,000	124,000	71,000
Brokerage, management fees and commissions	0	0	0	0
Bankcard income	1,000	0	2,000	1,000
Trust services and investment management	0	0	0	0
Securities gains (losses), net	0	0	0	0
Loss on debt extinguishment	0		0
Purchase accounting gain		0	0	0
Other income	9,000	2,000	19,000	12,000
Total noninterest income	142,000	181,000	476,000	410,000
Corporate
Noninterest income:
Fixed income	0	0	0	0
Deposit transactions and cash management	2,000	2,000	5,000	4,000
Mortgage banking and title income	1,000	0	2,000	1,000
Brokerage, management fees and commissions	0	0	0	0
Bankcard income	0	0	1,000	0
Trust services and investment management	0	0	0	0
Securities gains (losses), net	1,000	(1,000)	12,000	(2,000)
Loss on debt extinguishment	(23,000)		(23,000)
Purchase accounting gain		532,000	(1,000)	532,000
Other income	12,000	12,000	37,000	20,000
Total noninterest income	(7,000)	545,000	33,000	555,000
Underwriting, portfolio advisory, and other noninterest income
Noninterest income:
Revenue from contract with customer	$ 12,000	$ 10,000	$ 35,000	$ 29,000