Investment Securities - Investments in Debt and Equity Securities Reported Less than Historical Cost (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Fair value	$ 488,726	$ 932,577
Investments reported at less than historical cost
Schedule of Investments [Line Items]
Historical cost	494,102	951,571
Fair value	488,726	932,577
Gross unrealized losses	$ 5,376	$ 18,994
Percent of the Corporation's investment portfolio	26.20%	57.10%