Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	$ (200,542)
Amounts arising during the period	(733)
Amounts reclassified from AOCL to interest expense	(2)
Amounts reclassified from AOCL to accumulated deficit	23,265
Net change	22,530
Balance at end of period	(1,335,442)
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	992
Amounts reclassified from AOCL to interest expense	(2)
Amounts reclassified from AOCL to accumulated deficit	(150)
Net change	840
Balance at end of period	135
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	(3,105)
Amounts reclassified from AOCL to accumulated deficit	23,787
Net change	20,682
Balance at end of period	121,293
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	1,380
Amounts reclassified from AOCL to accumulated deficit	(372)
Net change	1,008
Balance at end of period	(4,047)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	94,851
Balance at end of period	$ 117,381