Retirement Benefit Plans (Obligation and Funded Status) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Amounts recognized in the balance sheets consist of:
Non-current liabilities	$ (8,838)	$ (8,807)
Pension Plan
Change in benefit obligation
Projected benefit obligation at beginning of year	41,854	42,026
Interest cost	1,285	1,710
Actuarial loss	3,996	3,121
Benefits paid	(2,535)	(2,391)
Settlements	0	(2,612)
Projected benefit obligation at end of year	44,600	41,854
Accumulated benefit obligation at end of year	44,600	41,854
Change in plan assets
Fair value of plan assets at beginning of year	37,364	34,954
Actual return on plan assets	5,763	6,930
Employer contributions	507	483
Benefits paid	(2,535)	(2,391)
Settlements	0	(2,612)
Fair value of plan assets at end of year	41,099	37,364
Funded status at end of year	(3,501)	(4,490)
Amounts recognized in the balance sheets consist of:
Non-current assets	4,070	3,079
Current liabilities	(549)	(606)
Non-current liabilities	(7,022)	(6,963)
Amount recognized in the balance sheets	(3,501)	(4,490)
Components of accumulated other comprehensive loss (income) consist of:
Actuarial loss	14,022	13,951
Prior service cost	761	819
Deferred taxes	(3,316)	(3,305)
Accumulated other comprehensive (loss) income	11,467	11,465
Other Postretirement Benefits Plan
Change in benefit obligation
Projected benefit obligation at beginning of year	2,049	2,113
Service cost	21	24
Interest cost	52	77
Plan amendments	49	0
Actuarial loss	145	46
Benefits paid	(262)	(211)
Projected benefit obligation at end of year	2,054	2,049
Change in plan assets
Funded status at end of year	(2,054)	(2,049)
Amounts recognized in the balance sheets consist of:
Current liabilities	(238)	(204)
Non-current liabilities	(1,816)	(1,845)
Amount recognized in the balance sheets	(2,054)	(2,049)
Components of accumulated other comprehensive loss (income) consist of:
Actuarial loss	466	227
Prior service cost	(167)	(259)
Deferred taxes	(167)	(96)
Accumulated other comprehensive (loss) income	$ 132	$ (128)