Pension plans and other postretirement benefits (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Pension benefits [Member]
Net periodic defined benefit cost for defined benefit plans
Service cost	$ 8,557	$ 4,918	$ 20,270	$ 14,753
Interest cost on projected benefit obligation	16,155	12,031	42,391	36,092
Expected return on plan assets	(16,718)	(12,722)	(43,981)	(38,165)
Amortization of prior service cost	(1,640)	(1,640)	(4,919)	(4,919)
Amortization of net actuarial loss	5,132	3,388	15,397	10,164
Net periodic benefit costs	11,486	5,975	29,158	17,925
Other postretirement benefits [Member]
Net periodic defined benefit cost for defined benefit plans
Service cost	148	101	388	303
Interest cost on projected benefit obligation	1,038	782	2,722	2,347
Amortization of prior service cost	27	44	81	132
Amortization of net actuarial loss	9	(2)	27	(7)
Net periodic benefit costs	$ 1,222	$ 925	$ 3,218	$ 2,775