Retirement Plans (Weighted-Average Asset Allocation for the Defined Benefit Cash Balance Plan) (Details)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations	100.00%	100.00%
Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations	32.00%	33.00%
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations	64.00%	64.00%
Other Liabilities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actual plan asset allocations	4.00%	3.00%