Retirement Benefit Plans (Summary of Net Periodic Benefit Costs) (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
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			$ 0	$ 0	$ 0
Interest cost on benefit obligations			6,511	7,941	7,212
Expected return on plan assets			(13,522)	(11,436)	(12,716)
Recognized net loss			4,027	5,705	4,862
Net periodic benefit cost (benefit)			(2,984)	2,210	(642)
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0			0
Interest cost on benefit obligations	46	65	46	65	103
Expected return on plan assets	0	0			0
Recognized net loss	23	14			2,846
Net periodic benefit cost (benefit)	69	79			2,949
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0			0
Interest cost on benefit obligations	46	85	$ 46	$ 85	78
Expected return on plan assets	0	0			0
Recognized net loss	(74)	(13)			0
Net periodic benefit cost (benefit)	$ (28)	$ 72			$ 78