Retirement Benefit Plans (Changes in Other Comprehensive Income) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	$ (2,202)	$ 9,600	$ (155)
Amounts reclassified from AOCL	(5,706)	(7,708)	(6,612)
Total (gain) loss recognized in OCI	(7,908)	1,892	(6,767)
Pension Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	(2,263)	9,952	(561)
Amounts reclassified from AOCL	(5,705)	(4,862)	(5,864)
Total (gain) loss recognized in OCI	(7,968)	5,090	(6,425)
SERP [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	164	0	1,037
Amounts reclassified from AOCL	(14)	(2,846)	(748)
Total (gain) loss recognized in OCI	150	(2,846)	289
Other Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net (gain) loss	(103)	(352)	(631)
Amounts reclassified from AOCL	13	0	0
Total (gain) loss recognized in OCI	$ (90)	$ (352)	$ (631)

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Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income for defined benefit plan.

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Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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