Pension and Other Postretirement Employee Benefits (Schedule of Pension and OPEB Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		$ 117,246	$ 110,160	$ 95,556	$ 111,445
Prior service credit (cost)	[1]	1,608	1,310	3,216	2,620
Actuarial loss	[1]	(3,290)	(2,292)	(6,623)	(4,887)
Total reclassification for the period	[1]	(1,682)	(982)	(3,407)	(2,267)
Amortization reclassified from AOCL	[2]			23,415
AOCI at end of period		115,564	109,178	115,564	109,178
Pension Plans [Member]
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		121,293	118,231	100,611	120,627
Prior service credit (cost)	[1]	(35)	(44)	(69)	(88)
Actuarial loss	[1]	(3,067)	(2,066)	(6,138)	(4,418)
Total reclassification for the period	[1]	(3,102)	(2,110)	(6,207)	(4,506)
Amortization reclassified from AOCL	[2]			23,787
AOCI at end of period		118,191	116,121	118,191	116,121
OPEB [Member]
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		(4,047)	(8,071)	(5,055)	(9,182)
Prior service credit (cost)	[1]	1,643	1,354	3,285	2,708
Actuarial loss	[1]	(223)	(226)	(485)	(469)
Total reclassification for the period	[1]	1,420	1,128	2,800	2,239
Amortization reclassified from AOCL	[2]			(372)
AOCI at end of period		$ (2,627)	$ (6,943)	$ (2,627)	$ (6,943)

[1] Amortization of prior service credit (cost) and amortization of actuarial loss are included in the computation of net periodic cost (benefit).

[2] See Note 2: Recent Accounting Pronouncements discussing the $23.4 million reclassification from AOCL to accumulated deficit.