Loans and Allowance for Credit Losses Allowance for Loan Losses by Portfolio Segment (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 364,462,000	$ 379,156,000	$ 379,156,000
Loans charged off	(18,318,000)	(20,034,000)
Recoveries of loans previously charged off	3,410,000	7,443,000
Net loans (charged off) recovered	(14,908,000)	(12,591,000)
Provision for credit losses	14,442,000	13,898,000
Ending balance	367,489,000	379,677,000	364,462,000
Loans - Excluding OBS Credit Exposure
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	364,462,000	379,156,000	379,156,000
Initial allowance for credit losses on purchased loans	3,351,000
Loans charged off	(18,318,000)	(20,034,000)
Recoveries of loans previously charged off	3,410,000	7,443,000
Net loans (charged off) recovered	(14,908,000)	(12,591,000)
Provision for credit losses	14,584,000	13,112,000
Ending balance	367,489,000	379,677,000	364,462,000
Loans - Excluding OBS Credit Exposure | Real estate - commercial mortgage
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	157,302,000	158,181,000	158,181,000
Initial allowance for credit losses on purchased loans	2,778,000
Loans charged off	(4,102,000)	(12,106,000)
Recoveries of loans previously charged off	701,000	374,000
Net loans (charged off) recovered	(3,401,000)	(11,732,000)
Provision for credit losses	2,363,000	15,697,000
Ending balance	159,042,000	162,146,000	157,302,000
Loans - Excluding OBS Credit Exposure | Commercial and Industrial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	77,740,000	92,212,000	92,212,000
Initial allowance for credit losses on purchased loans	494,000
Loans charged off	(10,545,000)	(3,865,000)
Recoveries of loans previously charged off	740,000	5,952,000
Net loans (charged off) recovered	(9,805,000)	2,087,000
Provision for credit losses	10,549,000	2,552,000
Ending balance	78,978,000	96,851,000	77,740,000
Loans - Excluding OBS Credit Exposure | Real estate - residential mortgage
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	88,961,000	81,331,000	81,331,000
Initial allowance for credit losses on purchased loans	0
Loans charged off	(391,000)	(343,000)
Recoveries of loans previously charged off	72,000	174,000
Net loans (charged off) recovered	(319,000)	(169,000)
Provision for credit losses	1,218,000	1,254,000
Ending balance	89,860,000	82,416,000	88,961,000
Loans - Excluding OBS Credit Exposure | Consumer and Real Estate - Home Equity
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	23,026,000	19,397,000	19,397,000
Initial allowance for credit losses on purchased loans	0
Loans charged off	(2,164,000)	(2,193,000)
Recoveries of loans previously charged off	584,000	660,000
Net loans (charged off) recovered	(1,580,000)	(1,533,000)
Provision for credit losses	2,748,000	1,430,000
Ending balance	24,194,000	19,294,000	23,026,000
Loans - Excluding OBS Credit Exposure | Real estate - construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	10,896,000	25,140,000	25,140,000
Initial allowance for credit losses on purchased loans	79,000
Loans charged off	0	0
Recoveries of loans previously charged off	884,000	82,000
Net loans (charged off) recovered	884,000	82,000
Provision for credit losses	(2,506,000)	(9,322,000)
Ending balance	9,353,000	15,900,000	10,896,000
Loans - Excluding OBS Credit Exposure | Leases and other loans
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,537,000	2,895,000	2,895,000
Initial allowance for credit losses on purchased loans	0
Loans charged off	(1,116,000)	(1,527,000)
Recoveries of loans previously charged off	429,000	201,000
Net loans (charged off) recovered	(687,000)	(1,326,000)
Provision for credit losses	212,000	1,501,000
Ending balance	$ 6,062,000	$ 3,070,000	$ 6,537,000