Employee Benefit Plan Obligations (Details 6) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Components of the net periodic benefit cost
Gain from pension plan curtailment			$ (4,132)
Pension Benefits [Member]
Components of the net periodic benefit cost
Service cost		1,614	2,861
Interest cost	4,578	4,339	4,001
Expected return on plan assets	(6,236)	(5,451)	(4,356)
Amortization of prior service cost			(392)
Recognized net actuarial loss	940	1,817	3,503
Gain from pension plan curtailment			(4,132)
Net periodic benefit cost	(718)	2,319	1,485
Postretirement Benefits [Member]
Components of the net periodic benefit cost
Service cost	555	465	412
Interest cost	1,285	1,213	1,155
Amortization of prior service cost	(543)	(511)	(511)
Recognized net actuarial loss	901	691	624
Net periodic benefit cost	$ 2,198	$ 1,858	$ 1,680