Retirement Benefit Plans (Changes in Other Comprehensive Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	$ 5,262	$ (2,202)	$ 9,600
Amounts reclassified from AOCL	(3,976)	(5,706)	(7,708)
Total (gain) loss recognized in OCI	1,286	(7,908)	1,892
Pension Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	5,375	(2,263)	9,952
Amounts reclassified from AOCL	(4,027)	(5,705)	(4,862)
Total (gain) loss recognized in OCI	1,348	(7,968)	5,090
SERP [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	194	164	0
Amounts reclassified from AOCL	(23)	(14)	(2,846)
Total (gain) loss recognized in OCI	171	150	(2,846)
Other Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	(307)	(103)	(352)
Amounts reclassified from AOCL	74	13	0
Total (gain) loss recognized in OCI	$ (233)	$ (90)	$ (352)