Other comprehensive income (loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Changes, net of income tax, in other comprehensive income
Balance at the beginning of the period	$ (28,173)	$ (25,609)
Balance at the beginning of the period, tax	18,741	18,998
Other comprehensive income (loss) before reclassifications	(427)	(3,474)
Amount reclassified from accumulated other comprehensive income (loss)
Amortization of prior service credit, net of tax	(125)	(137)
Amortization of prior service credit, tax	48	58
Amortization of actuarial losses, net of tax	334	829
Amortization of actuarial losses, tax	188	455
Net current-period other comprehensive income (loss)	(218)	(2,782)
Balance at the end of the period	(28,391)	(28,391)
Balance at the end of the period, tax	18,601	18,601
Pension and Other Postretirement Benefit Costs
Changes, net of income tax, in other comprehensive income
Balance at the beginning of the period	(29,902)	(30,385)
Amount reclassified from accumulated other comprehensive income (loss)
Amortization of prior service credit, net of tax	(125)	(137)
Amortization of actuarial losses, net of tax	334	829
Net current-period other comprehensive income (loss)	209	692
Balance at the end of the period	(29,693)	(29,693)
Foreign Currency Translation Adjustments
Changes, net of income tax, in other comprehensive income
Balance at the beginning of the period	1,729	4,776
Other comprehensive income (loss) before reclassifications	(427)	(3,474)
Amount reclassified from accumulated other comprehensive income (loss)
Net current-period other comprehensive income (loss)	(427)	(3,474)
Balance at the end of the period	$ 1,302	$ 1,302