Pension and Other Postretirement Employee Benefits (Schedule of Pension and OPEB Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		$ 115,564	$ 109,178	$ 95,556	$ 111,445
Prior service credit (cost)	[1]	1,608	1,309	4,824	3,929
Actuarial loss	[1]	(3,311)	(2,443)	(9,934)	(7,330)
Total reclassification for the period	[1]	(1,703)	(1,134)	(5,110)	(3,401)
Amortization reclassified from AOCL	[2]			23,415
AOCI at end of period		113,861	108,044	113,861	108,044
Pension Plans [Member]
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		118,191	116,121	100,611	120,627
Prior service credit (cost)	[1]	(34)	(44)	(103)	(132)
Actuarial loss	[1]	(3,069)	(2,209)	(9,207)	(6,627)
Total reclassification for the period	[1]	(3,103)	(2,253)	(9,310)	(6,759)
Amortization reclassified from AOCL	[2]			23,787
AOCI at end of period		115,088	113,868	115,088	113,868
OPEB [Member]
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		(2,627)	(6,943)	(5,055)	(9,182)
Prior service credit (cost)	[1]	1,642	1,353	4,927	4,061
Actuarial loss	[1]	(242)	(234)	(727)	(703)
Total reclassification for the period	[1]	1,400	1,119	4,200	3,358
Amortization reclassified from AOCL	[2]			(372)
AOCI at end of period		$ (1,227)	$ (5,824)	$ (1,227)	$ (5,824)

[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 $23.3 million reclassification from AOCL to accumulated deficit.