Investment Securities - Investments in Debt and Equity Securities Reported Less than Historical Cost (Details) - Investments reported at less than historical cost - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Historical cost	$ 816,240	$ 15,368
Fair value	800,079	15,019
Gross unrealized losses	$ 16,161	$ 349
Percent of the Corporation's investments available for sale	33.70%	0.80%