Pension plans and other postretirement benefits (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension Benefits [Member]
Net periodic defined benefit cost for defined benefit plans
Service cost	$ 28,828	$ 19,670	$ 19,483
Interest cost on benefit obligation	58,545	47,905	46,107
Expected return on plan assets	(60,700)	(50,844)	(46,976)
Amortization of prior service cost	(6,559)	(6,559)	(6,559)
Recognized net actuarial loss	20,530	13,551	8,292
Net periodic benefit costs	40,644	23,723	20,347
Other Postretirement Benefit Plans [Member]
Net periodic defined benefit cost for defined benefit plans
Service cost	535	383	353
Interest cost on benefit obligation	3,761	3,130	3,302
Amortization of prior service cost	107	176	243
Recognized net actuarial loss	36	(9)	(19)
Net periodic benefit costs	$ 4,439	$ 3,680	$ 3,879