Pension and Other Post-Employment Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 1,318	$ 1,026
Interest cost	2,699	2,692
Expected return on plan assets	(3,890)	(3,577)
Amortization of net actuarial losses	1,992	1,936
Total net periodic benefit cost	2,119	2,077
Other Benefits
Components of net periodic benefit cost
Service cost	93	96
Interest cost	425	470
Expected return on plan assets
Amortization of net actuarial losses	355	435
Total net periodic benefit cost	$ 873	$ 1,001