Non-pension Post-retirement Benefits (Amounts Recognized in Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Non-pension post-retirement benefits (current portion)	$ 4,185	$ 4,892
Non-pension post-retirement benefits	49,758	55,373
Non-pension Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Non-pension post-retirement benefits (current portion)	4,185	4,892
Non-pension post-retirement benefits	49,758	55,373
Total non-pension post-retirement benefits liability	$ 53,943	$ 60,265