Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	$ (200,542)
Amounts arising during the period	(3,471)
Amounts reclassified from AOCL to interest expense	(195)
Amounts reclassified from AOCL to accumulated deficit	23,265
Net change	19,599
Balance at end of period	(1,360,926)
Gains and losses on cash flow hedge [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(705)
Amounts arising during the period	(64)
Amounts reclassified from AOCL to interest expense	(195)
Amounts reclassified from AOCL to accumulated deficit	(150)
Net change	(409)
Balance at end of period	(1,114)
Pension and other postretirement employee benefits [Member] | Pension Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	100,611
Amounts arising during the period	(6,207)
Amounts reclassified from AOCL to accumulated deficit	23,787
Net change	17,580
Balance at end of period	118,191
Pension and other postretirement employee benefits [Member] | OPEB [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(5,055)
Amounts arising during the period	2,800
Amounts reclassified from AOCL to accumulated deficit	(372)
Net change	2,428
Balance at end of period	(2,627)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	94,851
Balance at end of period	$ 114,450