Pension and Other Postretirement Employee Benefits (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Loss [Roll Forward]
AOCL at beginning of period		$ 116,691	$ 97,454	$ 118,392	$ 98,720
Prior service cost (credit)	[1]	1,328	1,356	2,656	2,712
Actuarial loss	[1]	(2,974)	(2,452)	(6,003)	(5,074)
Total reclassification for the period	[1]	(1,646)	(1,096)	(3,347)	(2,362)
AOCL at end of period		115,045	96,358	115,045	96,358
Pension Plans [Member]
Accumulated Other Comprehensive Loss [Roll Forward]
Prior service cost (credit)	[1]	(92)	(114)	(184)	(228)
Actuarial loss	[1]	(2,689)	(2,200)	(5,378)	(4,408)
Total reclassification for the period	[1]	(2,781)	(2,314)	(5,562)	(4,636)
Other Postretirement Employee Benefits [Member]
Accumulated Other Comprehensive Loss [Roll Forward]
Prior service cost (credit)	[1]	1,420	1,470	2,840	2,940
Actuarial loss	[1]	(285)	(252)	(625)	(666)
Total reclassification for the period	[1]	$ 1,135	$ 1,218	$ 2,215	$ 2,274

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