Pension and other postretirement benefits (Details) (Postretirement Benefit Plan [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2009	Apr. 30, 2009	Dec. 31, 2008
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Minimum age for eligibility to receive the maximum available benefits, prior to amendment (in years)		58
Minimum age for eligibility to receive the maximum available benefits, amended (in years)		51
Service requirement for maximum retiree cost sharing, prior to amendment (in years)		30
Service requirement for maximum retiree cost sharing, amended (in years)		25
Average remaining service period of plan participants (in years)			8.2
Decrease in postretirement benefit expense as a result of change in amortization period	$ 5,208