Retirement Benefit Plans (Schedule of Changes in Plan Assets, Changes in Projected Benefit Obligation, Rate Assumptions, and Components of Net Periodic Benefit Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Change in fair value of plan assets
Fair value of plan assets at beginning of year	$ 17,858
Fair value of plan assets at end of year	20,327	$ 17,858
Pension Plans, Defined Benefit
Change in projected benefit obligation
Benefit obligation at beginning of year	20,787	30,210
Service cost	4	12
Interest cost	831	1,287
Liability actuarial (gain) loss	866	56
Benefits paid	(936)	(1,443)
Settlements		(9,335)
Benefit obligation at end of year	21,552	20,787
Change in fair value of plan assets
Fair value of plan assets at beginning of year	17,858	25,711
Return on plan assets	2,505	1,403
Employer contributions	900	1,522
Benefits paid	(936)	(1,443)
Settlements		(9,335)
Fair value of plan assets at end of year	20,327	17,858
Underfunded status	(1,225)	(2,929)
Supplemental Employee Retirement Plans, Defined Benefit
Change in projected benefit obligation
Benefit obligation at beginning of year	5,387	3,962
Service cost	198	176
Interest cost	204	175
Liability actuarial (gain) loss	247	183
Benefits paid	(91)	(67)
Plan amendments	0	958
Benefit obligation at end of year	5,945	5,387
Hourly Employees’ Pension Plan | Pension Plans, Defined Benefit
Change in projected benefit obligation
Benefit obligation at beginning of year	0	9,649
Service cost	0	8
Interest cost	0	401
Liability actuarial (gain) loss	0	(148)
Benefits paid	0	(575)
Settlements		(9,335)
Benefit obligation at end of year	0	0
Change in fair value of plan assets
Fair value of plan assets at beginning of year	0	9,019
Return on plan assets	0	119
Employer contributions	0	772
Benefits paid	0	(575)
Settlements		(9,335)
Fair value of plan assets at end of year	0	0
Underfunded status	0	0
Employees’ Retirement Plan | Pension Plans, Defined Benefit
Change in projected benefit obligation
Benefit obligation at beginning of year	20,787	20,561
Service cost	4	4
Interest cost	831	886
Liability actuarial (gain) loss	866	204
Benefits paid	(936)	(868)
Settlements		0
Benefit obligation at end of year	21,552	20,787
Change in fair value of plan assets
Fair value of plan assets at beginning of year	17,858	16,692
Return on plan assets	2,505	1,284
Employer contributions	900	750
Benefits paid	(936)	(868)
Settlements		0
Fair value of plan assets at end of year	20,327	17,858
Underfunded status	$ (1,225)	$ (2,929)