Retirement Benefit Plans (Changes in Plan Assets and Funded Status of Pension Plans and Other Postretirement Benefit Plans) (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets:
Beginning balance			$ 239,621
Ending balance			266,268	$ 239,621
Funded status of the plan at year end (2)			2,600
Accumulated benefit obligation			270,500	245,700
Pension Plan [Member]
Change in benefit obligation:
Beginning balance	$ 209,513		241,404	209,513
Interest cost			6,511	7,941	$ 7,212
Actuarial loss (gain)			(27,376)	(33,157)
Benefits paid and administrative expenses			(8,877)	(9,207)
Ending balance (1)			266,414	241,404	209,513
Change in plan assets:
Beginning balance	191,972		239,621	191,972
Actual return on plan assets			35,524	46,856
Employer contributions			0	10,000
Benefits paid and administrative expenses			(8,877)	(9,207)
Ending balance			266,268	239,621	191,972
Funded status of the plan at year end (2)			(146)	(1,783)
SERP [Member]
Change in benefit obligation:
Beginning balance	1,835		1,935	1,835
Interest cost	46	$ 65	46	65	103
Actuarial loss (gain)			(194)	(163)
Benefits paid and administrative expenses			(129)	(128)
Ending balance (1)			2,046	1,935	1,835
Change in plan assets:
Beginning balance	0		0	0
Actual return on plan assets			0	0
Employer contributions			129	128
Benefits paid and administrative expenses			(129)	(128)
Ending balance			0	0	0
Funded status of the plan at year end (2)			(2,046)	(1,935)
Other Benefits [Member]
Change in benefit obligation:
Beginning balance	2,612		2,399	2,612
Interest cost	46	$ 85	46	85	78
Actuarial loss (gain)			307	103
Benefits paid and administrative expenses			(140)	(195)
Ending balance (1)			1,998	2,399	2,612
Change in plan assets:
Beginning balance	$ 0		0	0
Actual return on plan assets			0	0
Employer contributions			140	195
Benefits paid and administrative expenses			(140)	(195)
Ending balance			0	0	$ 0
Funded status of the plan at year end (2)			$ (1,998)	$ (2,399)