Pension plans and other postretirement benefits (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension Benefits [Member]
Net periodic defined benefit cost for defined benefit plans
Service cost	$ 6,875	$ 6,413	$ 14,775	$ 11,713
Interest cost on projected benefit obligation	15,418	14,086	31,018	26,236
Expected return on plan assets	(17,581)	(14,563)	(35,256)	(27,263)
Amortization of prior service cost	(1,629)	(1,629)	(3,279)	(3,279)
Amortization of net actuarial loss	9,183	5,165	18,583	10,265
Net periodic benefit cost	12,266	9,472	25,841	17,672
Other Postretirement Benefit Plans [Member]
Net periodic defined benefit cost for defined benefit plans
Service cost	159	115	334	240
Interest cost on projected benefit obligation	919	909	1,869	1,684
Amortization of prior service cost	10	29	10	54
Amortization of net actuarial loss	165	18	265	18
Net periodic benefit cost	$ 1,253	$ 1,071	$ 2,478	$ 1,996