Business Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Fixed income	$ 102,000	$ 112,000	$ 228,000	$ 208,000
Mortgage banking and title income	38,000	4,000	91,000	7,000
Deposit transactions and cash management	44,000	31,000	86,000	61,000
Brokerage, management fees and commissions	21,000	14,000	41,000	29,000
Trust services and investment management	14,000	8,000	26,000	15,000
Bankcard income	15,000	7,000	25,000	14,000
Securities gains (losses), net	11,000	(1,000)	11,000	(1,000)
Other income	40,000	31,000	75,000	48,000
Total noninterest income	285,000	206,000	583,000	381,000
Regional Banking
Disaggregation of Revenue [Line Items]
Fixed income	0	0	1,000	0
Mortgage banking and title income	0	0	0	0
Deposit transactions and cash management	39,000	27,000	77,000	52,000
Brokerage, management fees and commissions	21,000	14,000	41,000	29,000
Trust services and investment management	14,000	8,000	26,000	15,000
Bankcard income	14,000	7,000	24,000	13,000
Securities gains (losses), net	0	0	0	0
Other income	20,000	13,000	39,000	33,000
Total noninterest income	108,000	69,000	208,000	142,000
Specialty Banking
Disaggregation of Revenue [Line Items]
Fixed income	102,000	112,000	227,000	208,000
Mortgage banking and title income	38,000	4,000	90,000	6,000
Deposit transactions and cash management	3,000	3,000	6,000	6,000
Brokerage, management fees and commissions	0	0	0	0
Trust services and investment management	0	0	0	0
Bankcard income	1,000	0	1,000	1,000
Securities gains (losses), net	0	0	0	0
Other income	6,000	5,000	11,000	7,000
Total noninterest income	150,000	124,000	335,000	228,000
Corporate
Disaggregation of Revenue [Line Items]
Fixed income	0	0	0	0
Mortgage banking and title income	0	0	1,000	1,000
Deposit transactions and cash management	2,000	1,000	3,000	3,000
Brokerage, management fees and commissions	0	0	0	0
Trust services and investment management	0	0	0	0
Bankcard income	0	0	0	0
Securities gains (losses), net	11,000	(1,000)	11,000	(1,000)
Other income	14,000	13,000	25,000	8,000
Total noninterest income	27,000	13,000	40,000	11,000
Underwriting, portfolio advisory, and other noninterest income
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 14,000	$ 9,000	$ 24,000	$ 18,000