Employee Benefit Plans - Supplemental Non-qualified Unfunded Plans (Narrative) (Details) - Officers and Key Employees - Other postretirement benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Compensation expense	$ 0.2	$ 0.3	$ 0.2
Amount accrued	$ 2.1	$ 1.9