Accumulated Other Comprehensive Loss (Schedule of Reclassifications Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Reclassifciations Out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) before income taxes		$ 29,155	$ (19,065)	$ 26,863	$ (24,635)
Income tax provision (benefit)		(7,742)	(7,652)	(7,432)	(5,117)
Net income (loss)		21,413	$ (26,717)	19,431	$ (29,752)
Reclassification out of Accumulated Other Comprehensive Income
Schedule of Reclassifciations Out of Accumulated Other Comprehensive Income [Line Items]
Net income (loss)	[1]	(168)		(336)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of prior-service cost
Schedule of Reclassifciations Out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior-service cost	[2],[3]	(228)		(456)
Income (loss) before income taxes	[1]	(228)		(456)
Income tax provision (benefit)	[1]	60		120
Net income (loss)	[1]	$ (168)		$ (336)

[1] Amounts in parentheses indicate credits to profit/loss

[2] All amounts are net of tax. Amounts in parentheses indicate debits

[3] This accumulated other comprehensive loss component is included in the computation of net periodic benefit cost (see Note 6).