Corporate Premises and Equipment - Classifications of Corporate Premises and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment
Corporate premises and equipment, Gross	$ 76,412	$ 73,946
Less accumulated depreciation	(39,443)	(38,142)
Corporate premises and equipment, net	36,969	35,804
Land
Property, Plant and Equipment
Corporate premises and equipment, Gross	8,340	8,340
Building
Property, Plant and Equipment
Corporate premises and equipment, Gross	35,586	35,352
Equipment, Furniture and Fixtures
Property, Plant and Equipment
Corporate premises and equipment, Gross	$ 32,486	$ 30,254