Investment Securities - Investments in Debt and Equity Securities Reported Less than Historical Cost (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Fair value	$ 318,190	$ 510,297
Investments reported at less than historical cost
Schedule of Investments [Line Items]
Historical cost	326,043	523,773
Fair value	$ 318,190	$ 510,297
Percent of the Corporation's investment portfolio	23.70%	39.10%