Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at January 1			$ (156,274)
Other comprehensive (income) loss before reclassifications			(2,039)
Amounts reclassified from accumulated other comprehensive loss			(91)
Net current period other comprehensive (income) loss	$ (797)	$ (1,170)	(2,130)	$ (2,381)
Balance at June 30	(170,329)		(170,329)
Gains and losses on cash flow hedge [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at January 1			(701)
Other comprehensive (income) loss before reclassifications			228
Amounts reclassified from accumulated other comprehensive loss			(91)
Net current period other comprehensive (income) loss			137
Balance at June 30	(564)		(564)
Pension and other postretirement employee benefits [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at January 1			111,445
Other comprehensive (income) loss before reclassifications			(2,267)
Net current period other comprehensive (income) loss			(2,267)
Balance at June 30	109,178		109,178
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at January 1			110,744
Balance at June 30	$ 108,614		$ 108,614