Investment Securities - Investments in Debt and Equity Securities Reported Less than Historical Cost (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Fair value	$ 990,539	$ 446,047
Investments reported at less than historical cost
Schedule of Investments [Line Items]
Historical cost	1,013,210	452,644
Fair value	$ 990,539	$ 446,047
Percent of the Corporation's investment portfolio	64.20%	28.60%