Benefit Plans, Investment Strategies (Details)	12 Months Ended
	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Average rate of return on assets	9.00%
Equity Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target long-term investment portfolio mix	55.00%
Equity Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target long-term investment portfolio mix	65.00%
Fixed Income Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target long-term investment portfolio mix	30.00%
Fixed Income Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target long-term investment portfolio mix	35.00%
Other Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target long-term investment portfolio mix	0.00%
Other Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target long-term investment portfolio mix	15.00%