Investment Securities - Investments in Debt and Equity Securities Reported Less than Historical Cost (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Fair value	$ 433,072	$ 932,577
Investments reported at less than historical cost
Schedule of Investments [Line Items]
Historical cost	435,367	951,571
Fair value	433,072	932,577
Gross unrealized losses	$ 2,295	$ 18,994
Percent of the Corporation's investment portfolio	17.40%	57.10%