Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	$ (156,274)	$ (203,736)	$ (225,066)
Amounts arising during the period	(11,210)	1,941
Amounts reclassified from AOCL	(4,683)	(5,700)
Net change	(15,893)	(3,759)	(3,889)
Balance at end of period	(200,542)	(156,274)	(203,736)
Gains and losses on cash flow hedge [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(701)
Amounts arising during the period	145	(916)
Amounts reclassified from AOCL	(149)	215
Net change	(4)	(701)
Balance at end of period	(705)	(701)
Pension and other postretirement employee benefits [Member] | Pension Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	120,627	128,244
Amounts arising during the period	(11,005)	2,666
Amounts reclassified from AOCL	(9,011)	(10,283)
Net change	(20,016)	(7,617)
Balance at end of period	100,611	120,627	128,244
Pension and other postretirement employee benefits [Member] | OPEB [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(9,182)	(13,741)
Amounts arising during the period	(350)	191
Amounts reclassified from AOCL	4,477	4,368
Net change	4,127	4,559
Balance at end of period	(5,055)	(9,182)	(13,741)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	110,744	114,503	118,392
Balance at end of period	$ 94,851	$ 110,744	$ 114,503