Pension and Other Postretirement Employee Benefits (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Loss [Roll Forward]
AOCL at beginning of period		$ 115,045	$ 96,358	$ 118,392	$ 98,720
Prior service credit (cost)	[1]	1,327	1,357	3,983	4,069
Actuarial loss	[1]	(3,139)	(2,538)	(9,142)	(7,612)
Total reclassification for the period	[1]	(1,812)	(1,181)	(5,159)	(3,543)
AOCL at end of period		113,233	95,177	113,233	95,177
Pension Plans [Member]
Accumulated Other Comprehensive Loss [Roll Forward]
Prior service credit (cost)	[1]	(93)	(114)	(277)	(342)
Actuarial loss	[1]	(2,828)	(2,204)	(8,206)	(6,612)
Total reclassification for the period	[1]	(2,921)	(2,318)	(8,483)	(6,954)
Other Postretirement Employee Benefits [Member]
Accumulated Other Comprehensive Loss [Roll Forward]
Prior service credit (cost)	[1]	1,420	1,471	4,260	4,411
Actuarial loss	[1]	(311)	(334)	(936)	(1,000)
Total reclassification for the period	[1]	$ 1,109	$ 1,137	$ 3,324	$ 3,411

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