Benefit Plans, Weighted Average Asset Allocation by Asset Category (Details)	Dec. 31, 2017	Dec. 31, 2016
Weighted average asset allocations [Abstract]
Weighted average asset allocation percentages, Total	100.00%	100.00%
Equity Securities [Member]
Weighted average asset allocations [Abstract]
Weighted average asset allocation percentages, Total	56.00%	60.20%
Fixed Income Securities [Member]
Weighted average asset allocations [Abstract]
Weighted average asset allocation percentages, Total	36.20%	32.70%
Real Estate [Member]
Weighted average asset allocations [Abstract]
Weighted average asset allocation percentages, Total	0.80%	0.70%
Other [Member]
Weighted average asset allocations [Abstract]
Weighted average asset allocation percentages, Total	7.00%	6.40%