Pension and Other Postretirement Employee Benefits (Change in Accumulated Other Comprehensive Loss (AOCL) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		$ 111,445	$ 114,503
Prior service credit (cost)	[1]	1,310	1,275
Actuarial loss	[1]	(2,595)	(2,650)
Total reclassification for the period	[1]	(1,285)	(1,375)
AOCI at end of period		110,160	113,128
Pension Plans [Member]
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		120,627	128,244
Prior service credit (cost)	[1]	(44)	(79)
Actuarial loss	[1]	(2,352)	(2,389)
Total reclassification for the period	[1]	(2,396)	(2,468)
AOCI at end of period		118,231	125,776
Other Postretirement Employee Benefits [Member]
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		(9,182)	(13,741)
Prior service credit (cost)	[1]	1,354	1,354
Actuarial loss	[1]	(243)	(261)
Total reclassification for the period	[1]	1,111	1,093
AOCI at end of period		$ (8,071)	$ (12,648)

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