Investment Securities - Investments in Debt and Equity Securities Reported Less than Historical Cost (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Fair value	$ 156,285	$ 932,577
Investments reported at less than historical cost
Schedule of Investments [Line Items]
Historical cost	157,406	951,571
Fair value	156,285	932,577
Gross unrealized losses	$ 1,121	$ 18,994
Percent of the Corporation's investment portfolio	7.50%	57.10%