Pension Benefit and Retirement Health and Life Insurance Benefits (Amounts Recognized in Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	$ 3,021	$ 2,583
Current liabilities	(5)	0
Noncurrent liabilities	(8,720)	(8,501)
Net amount recognized at end of year	(5,704)	(5,918)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net actuarial (loss) gain	(59,645)	(59,377)
Prior service benefit	0	0
Net amount recognized at end of year	(59,645)	(59,377)
Retirement Health and Life Insurance Benefits
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	0	0
Current liabilities	(352)	(512)
Noncurrent liabilities	(1,685)	(1,992)
Net amount recognized at end of year	(2,037)	(2,504)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net actuarial (loss) gain	63	523
Prior service benefit	2,821	3,878
Net amount recognized at end of year	$ 2,884	$ 4,401