Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable, Past Due [Line Items]
Total	$ 7,515,918	$ 7,183,969
Nonaccrual	12,677	19,052	$ 66,291
Current
Financing Receivable, Past Due [Line Items]
Total	7,488,899	7,144,576
Total Past Due Loans
Financing Receivable, Past Due [Line Items]
Total	27,019	39,393
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total	10,328	18,093
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total	4,014	2,248
Commercial Loans | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total	2,591,189	2,538,839
Nonaccrual	1,262	7,100	31,488
Commercial Loans | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total	1,424,141	1,510,392
Nonaccrual	919	283	15,239
Commercial Loans | Commercial construction
Financing Receivable, Past Due [Line Items]
Total	372,839	381,963
Nonaccrual	384	384	2,471
Commercial Loans | Business banking
Financing Receivable, Past Due [Line Items]
Total	1,363,314	1,205,944
Nonaccrual	4,010	4,490	9,641
Commercial Loans | Current | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total	2,589,832	2,523,315
Commercial Loans | Current | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total	1,423,222	1,505,805
Commercial Loans | Current | Commercial construction
Financing Receivable, Past Due [Line Items]
Total	366,269	381,579
Commercial Loans | Current | Business banking
Financing Receivable, Past Due [Line Items]
Total	1,356,627	1,199,586
Commercial Loans | Total Past Due Loans | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total	1,357	15,524
Commercial Loans | Total Past Due Loans | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total	919	4,587
Commercial Loans | Total Past Due Loans | Commercial construction
Financing Receivable, Past Due [Line Items]
Total	6,570	384
Commercial Loans | Total Past Due Loans | Business banking
Financing Receivable, Past Due [Line Items]
Total	6,687	6,358
Commercial Loans | 30-59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total	0	8,424
Commercial Loans | 30-59 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total	0	4,304
Commercial Loans | 30-59 Days Past Due | Commercial construction
Financing Receivable, Past Due [Line Items]
Total	6,186	0
Commercial Loans | 30-59 Days Past Due | Business banking
Financing Receivable, Past Due [Line Items]
Total	1,826	1,583
Commercial Loans | 60-89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total	95	0
Commercial Loans | 60-89 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total	0	0
Commercial Loans | 60-89 Days Past Due | Commercial construction
Financing Receivable, Past Due [Line Items]
Total	0	0
Commercial Loans | 60-89 Days Past Due | Business banking
Financing Receivable, Past Due [Line Items]
Total	851	285
Consumer Loans | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total	1,649,056	1,421,953
Nonaccrual	5,768	6,526	7,294
Consumer Loans | Other consumer
Financing Receivable, Past Due [Line Items]
Total	115,379	124,878
Nonaccrual	334	269	$ 158
Consumer Loans | Current | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total	1,638,157	1,409,907
Consumer Loans | Current | Other consumer
Financing Receivable, Past Due [Line Items]
Total	114,792	124,384
Consumer Loans | Total Past Due Loans | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total	10,899	12,046
Consumer Loans | Total Past Due Loans | Other consumer
Financing Receivable, Past Due [Line Items]
Total	587	494
Consumer Loans | 30-59 Days Past Due | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total	2,104	3,617
Consumer Loans | 30-59 Days Past Due | Other consumer
Financing Receivable, Past Due [Line Items]
Total	212	165
Consumer Loans | 60-89 Days Past Due | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total	3,027	1,903
Consumer Loans | 60-89 Days Past Due | Other consumer
Financing Receivable, Past Due [Line Items]
Total	$ 41	$ 60