Retirement and Employee Benefit Plans (Schedule of Assumptions Used - Benefit Obligations) (Details)	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.75%	4.20%
Rate of compensation increase	3.50%	3.50%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.75%	4.20%