Retirement Benefit Plans (Weighted Average Assumptions used to Determine Net Periodic Benefit Cost) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Assumed healthcare cost trend (as a percent)	7.50%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate (as a percent)	4.01%	4.20%	3.85%
Expected long-term return on assets (as a percent)	6.50%	7.00%	7.00%
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate (as a percent)	3.63%	3.75%	3.50%
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate (as a percent)	3.27%	3.35%	3.15%
Assumed healthcare cost trend (as a percent)	7.50%			8.25%	8.00%