Pension and other postretirement benefits (actuarial assumptions) (Details) (USD $) In Thousands, unless otherwise specified	4 Months Ended	8 Months Ended		12 Months Ended
	Apr. 29, 2009	Dec. 31, 2009		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Postretirement Benefit Plan [Member]
Assumptions used in calculating benefit obligation [Abstract]
Discount rate (in hundredths)	7.25%			4.20%	4.90%		6.60%
Assumptions used in calculating net periodic benefit expense [Abstract]
Discount rate (in hundredths)	6.60%	7.25%	[1]	4.90%	5.45%
Expected return on plan assets (in hundredths)				7.75%	8.00%	8.50%
Assumed health care cost trend rates [Abstract]
Health care cost trend rate assumed for next year (in hundredths)				7.50%	7.75%	8.00%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) (in hundredths)				5.00%	5.00%	5.00%
Year that the rate reaches the ultimate trend rate		2017		2017	2017	2017
Effect of one-percentage-point change in assumed health care cost trend rate [Abstract]
Effect of one-percentage-point increase on total of service and interest cost				$ 92
Effect of one-percentage-point decrease on total of service and interest cost				(87)
Effect of one-percentage-point increase on benefit obligation				2,185
Effect of one-percentage-point decrease on benefit obligation				$ (2,069)
Pension Plans [Member]
Assumptions used in calculating benefit obligation [Abstract]
Discount rate (in hundredths)				4.20%	4.90%
Assumptions used in calculating net periodic benefit expense [Abstract]
Discount rate (in hundredths)				4.90%	5.45%
Discount rate, range minimum (in hundredths)						4.06%
Discount rate, range maximum (in hundredths)						6.60%
Expected return on plan assets (in hundredths)				0.00%	0.00%	4.50%

[1] For 2009, the rate presented for our postretirement benefit plan was used from April 30, 2009 through December 31, 2009, the period subsequent to the 2009 plan amendments. A discount rate of 6.60% was used for the period from January 1, 2009 through April 30, 2009.