Retirement Benefit Plans (Changes in Other Comprehensive Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	$ (155)	$ 368	$ 8,719
Amounts reclassified from AOCL	(6,612)	(7,126)	(6,161)
Total (gain) loss recognized in OCI	(6,767)	(6,772)	2,485
Pension Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	(561)	(715)	8,525
Amounts reclassified from AOCL	(5,864)	(6,665)	(5,724)
Amortization of prior service cost	0	0	0
Total (gain) loss recognized in OCI	(6,425)	(7,380)	2,801
SERP [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	1,037	1,023	372
Amounts reclassified from AOCL	(748)	(426)	(390)
Amortization of prior service cost	0	0	0
Total (gain) loss recognized in OCI	289	597	(18)
Other Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	(631)	60	(178)
Amounts reclassified from AOCL	0	(35)	(47)
Amortization of prior service cost	0	(14)	(73)
Total (gain) loss recognized in OCI	$ (631)	$ 11	$ (298)