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Investment Securities - Investments in Debt and Equity Securities Reported Less than Historical Cost (Details) - Investments reported at less than historical cost - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Historical cost	$ 809,154	$ 15,368
Fair value	789,968	15,019
Gross unrealized losses	$ 19,186	$ 349
Percent of the Corporation's investments available for sale	21.40%	0.50%