Investment Securities - Investments in Debt and Equity Securities Reported Less than Historical Cost (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Fair value	$ 156,394	$ 395,416
Investments reported at less than historical cost
Schedule of Investments [Line Items]
Historical cost	157,859	398,172
Fair value	156,394	395,416
Gross unrealized losses	$ 1,465	$ 2,756
Percent of the Corporation's investment portfolio	9.40%	15.20%