Other comprehensive income (accumulated other comprehensive loss) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
Accumulated other comprehensive loss [Line Items]
Accumulated other comprehensive loss, beginning of year	$ (33,424)
Other comprehensive income (loss) before reclassifications	(3,526)
Amounts reclassified from accumulated other comprehensive loss	1,860
Net current-period other comprehensive income (loss)	(1,666)
Accumulated other comprehensive loss, end of period	(35,090)
Postretirement benefit plans, net of tax [Member]
Accumulated other comprehensive loss [Line Items]
Accumulated other comprehensive loss, beginning of year	(34,874)
Other comprehensive income (loss) before reclassifications	0
Amounts reclassified from accumulated other comprehensive loss	1,079
Net current-period other comprehensive income (loss)	1,079
Accumulated other comprehensive loss, end of period	(33,795)
Loss on derivatives, net of tax [Member]
Accumulated other comprehensive loss [Line Items]
Accumulated other comprehensive loss, beginning of year	(781)	[1]
Other comprehensive income (loss) before reclassifications	0	[1]
Amounts reclassified from accumulated other comprehensive loss	781	[1]
Net current-period other comprehensive income (loss)	781	[1]
Accumulated other comprehensive loss, end of period	0	[1]
Net unrealized loss on marketable securities, net of tax [Member]
Accumulated other comprehensive loss [Line Items]
Accumulated other comprehensive loss, beginning of year	(276)
Other comprehensive income (loss) before reclassifications	88	[2]
Amounts reclassified from accumulated other comprehensive loss	0
Net current-period other comprehensive income (loss)	88
Accumulated other comprehensive loss, end of period	(188)
Income tax expense related to other comprehensive income before reclassifications	31
Currency translation adjustment [Member]
Accumulated other comprehensive loss [Line Items]
Accumulated other comprehensive loss, beginning of year	2,507
Other comprehensive income (loss) before reclassifications	(3,614)
Amounts reclassified from accumulated other comprehensive loss	0
Net current-period other comprehensive income (loss)	(3,614)
Accumulated other comprehensive loss, end of period	$ (1,107)

[1] Relates to interest rate locks executed in 2004. See the caption "Note 6: Derivative financial instruments" in the Notes to Consolidated Financial Statements appearing in the 2013 Form 10-K.

[2] Other comprehensive income before reclassifications is net of income tax expense of $31.