Summary of Significant Accounting Policies (Details) $ in Thousands		9 Months Ended
		Sep. 30, 2023 USD ($) Segment	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of reportable segments | Segment		1
Decrease in retained earnings		$ (528,711)	$ (525,888)	$ (510,793)	$ (512,099)	$ (499,105)	$ (495,835)	$ (494,161)	$ (490,076)
Deferred income taxes		25,509			13,674
Off-balance-sheet credit exposure		4,500			85
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loan losses					15,000
Decrease in retained earnings	[1]				11,548
Deferred income taxes					3,100
Off-balance-sheet credit exposure					3,100
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in retained earnings		$ (137,199)	$ (131,271)	$ (129,617)	(133,114)	$ (120,572)	$ (108,265)	$ (99,206)	$ (91,043)
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in retained earnings	[1]				$ 11,548

[1] The Corporation adopted ASU 2016-13 "Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments" effective January 1, 2023. See "Note 1 - Summary of Significant Accounting Policies" for further details.