Other comprehensive income (loss) (accumulated other comprehensive loss) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Accumulated other comprehensive loss [Line Items]
Amounts reclassified from accumulated other comprehensive loss		$ 330	$ 139
Postretirement benefit plans [Member]
Accumulated other comprehensive loss [Line Items]
Balance, December 31, 2020		(21,956)
Other comprehensive (loss) income before reclassifications		0
Amounts reclassified from accumulated other comprehensive loss		83	208
Net current-period other comprehensive income (loss)		83
Balance, March 31, 2021		(21,873)
Net unrealized loss on available-for-sale debt securities [Member]
Accumulated other comprehensive loss [Line Items]
Balance, December 31, 2020		(90)
Other comprehensive (loss) income before reclassifications	[1]	(204)
Amounts reclassified from accumulated other comprehensive loss		0
Net current-period other comprehensive income (loss)		(204)
Balance, March 31, 2021		(294)
Unrealized loss on securities arising during the period, tax		(71)
Net unrealized loss on cash flow hedge [Member]
Accumulated other comprehensive loss [Line Items]
Balance, December 31, 2020		(5,351)
Other comprehensive (loss) income before reclassifications	[2]	477
Amounts reclassified from accumulated other comprehensive loss		247	$ (69)
Net current-period other comprehensive income (loss)		724
Balance, March 31, 2021		(4,627)
Unrealized gain on cash flow hedge arising during the period, tax		168
Currency translation adjustment [Member]
Accumulated other comprehensive loss [Line Items]
Balance, December 31, 2020		(14,036)
Other comprehensive (loss) income before reclassifications		1,006
Amounts reclassified from accumulated other comprehensive loss		0
Net current-period other comprehensive income (loss)		1,006
Balance, March 31, 2021		(13,030)
Accumulated other comprehensive loss [Member]
Accumulated other comprehensive loss [Line Items]
Balance, December 31, 2020		(41,433)
Other comprehensive (loss) income before reclassifications		1,279
Amounts reclassified from accumulated other comprehensive loss		330
Net current-period other comprehensive income (loss)		1,609
Balance, March 31, 2021		$ (39,824)

[1] Other comprehensive loss before reclassifications is net of an income tax benefit of $71.

[2] Other comprehensive income before reclassifications is net of income tax expense of $168.