Retirement Benefit Plans (Obligation and Funded Status) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized in the balance sheets consist of:
Non-current liabilities	$ (11,827)	$ (14,271)
Pension Plan
Change in benefit obligation
Projected benefit obligation at beginning of year	45,318	43,559
Interest cost	1,746	1,757	$ 1,713
Actuarial loss (gain)	1,275	2,134
Benefits paid	(2,019)	(1,811)
Intercompany transfers	255	321
Projected benefit obligation at end of year	46,065	45,318	43,559
Accumulated benefit obligation at end of year	46,065	45,318
Change in plan assets
Fair value of plan assets at beginning of year	34,628	37,162
Actual return on plan assets	5,461	(1,157)
Employer contributions	712	755
Benefits paid	(2,019)	(1,811)
Intercompany transfers	(255)	(321)
Fair value of plan assets at end of year	38,527	34,628	37,162
Funded status at end of year	(7,538)	(10,690)
Amounts recognized in the balance sheets consist of:
Noncurrent assets	2,051	1,387
Current liabilities	(700)	(721)
Non-current liabilities	(8,889)	(11,356)
Amount recognized in the balance sheets	(7,538)	(10,690)
Components of accumulated other comprehensive loss (income) consist of:
Actuarial loss	15,363	17,145
Prior service cost	937	995
Deferred taxes	(6,481)	(7,127)
Accumulated other comprehensive (loss) income	9,819	11,013
Other Postretirement Benefits Plan
Change in benefit obligation
Projected benefit obligation at beginning of year	3,211	3,466
Service cost	50	70	70
Interest cost	101	116	113
Actuarial loss (gain)	154	(25)
Benefits paid	(295)	(416)
Projected benefit obligation at end of year	3,221	3,211	$ 3,466
Change in plan assets
Funded status at end of year	(3,221)	(3,211)
Amounts recognized in the balance sheets consist of:
Current liabilities	(282)	(294)
Non-current liabilities	(2,939)	(2,917)
Amount recognized in the balance sheets	(3,221)	(3,211)
Components of accumulated other comprehensive loss (income) consist of:
Actuarial loss	1,040	983
Prior service cost	(78)	(95)
Deferred taxes	287	284
Accumulated other comprehensive (loss) income	$ 1,249	$ 1,172