Pension Plans and Other Postretirement Employee Benefits (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Accumulated Other Comprehensive Loss [Roll Forward]
AOCL at beginning of period	$ 97,454		$ 138,747		$ 98,720		$ 140,898
Prior service cost (credit)	1,356	[1]	1,361	[1]	2,712	[1]	2,723	[1]
Actuarial loss	(2,452)	[1]	(3,544)	[1]	(5,074)	[1]	(7,057)	[1]
Total reclassification for the period	(1,096)	[1]	(2,183)	[1]	(2,362)	[1]	(4,334)	[1]
AOCL at end of period	96,358		136,564		96,358		136,564
Pension Plans [Member]
Accumulated Other Comprehensive Loss [Roll Forward]
Prior service cost (credit)	(114)	[1]	(119)	[1]	(228)	[1]	(238)	[1]
Actuarial loss	(2,200)	[1]	(3,063)	[1]	(4,408)	[1]	(6,078)	[1]
Total reclassification for the period	(2,314)	[1]	(3,182)	[1]	(4,636)	[1]	(6,316)	[1]
Other Postretirement Employee Benefits [Member]
Accumulated Other Comprehensive Loss [Roll Forward]
Prior service cost (credit)	1,470	[1]	1,480	[1]	2,940	[1]	2,961	[1]
Actuarial loss	(252)	[1]	(481)	[1]	(666)	[1]	(979)	[1]
Total reclassification for the period	$ 1,218	[1]	$ 999	[1]	$ 2,274	[1]	$ 1,982	[1]

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