Retirement Benefit Plans (Summary of Components of Accumulated Other Comprehensive Loss Related to Pensions and Postretirement Benefits) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) included in AOCI	$ 57,902	$ 56,555
Deferred tax benefit	12,881	12,528
Amounts included in accumulated AOCL, net of tax	45,021	44,027
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) included in AOCI	773	602
Deferred tax benefit	172	133
Amounts included in accumulated AOCL, net of tax	601	469
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Amounts included in accumulated AOCL, net of tax	(537)	(357)
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) included in AOCI	(690)	(458)
Deferred tax benefit	$ (153)	$ (101)