Benefit Plans (Effects of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates) (Details) - Postretirement Benefit Plan [Member]	12 Months Ended
	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Effect on total of service and interest cost, One-Percentage-Point Increase	7.80%
Effect on total of service and interest cost, One-Percentage-Point Decrease	(6.90%)
Effect on postretirement benefit obligation, One-Percentage-Point Increase	6.60%
Effect on postretirement benefit obligation, One-Percentage-Point Decrease	(5.50%)