Employee Benefits (Weighted-Average Actuarial Assumptions for Benefit Obligations - Retirement Plans) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Effect of One Percentage Point Increase on Discount Rate	1.00%
Defined Benefit Plan, Decrease in Projected Benefit Obiligation, Percentage	12.40%
Defined Benefit Plan, Effect of One Percentage Point Decrease on Discount Rate	1.00%
Defined Benefit Plan, Increase in Projected Benefit Obiligation, Percentage	15.60%
Retirement Income Plan | Benefit Obligations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.77%	4.29%
Rate of compensation increase	4.50%	4.50%
Supplemental Employee Retirement Plan | Benefit Obligations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.40%	3.76%
Excess Benefit Plan | Benefit Obligations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.81%	4.34%
Rate of compensation increase	4.50%	4.50%