Other comprehensive (loss) income (accumulated other comprehensive loss) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year		$ 570,861	$ 915,413
Amounts reclassified from accumulated other comprehensive loss		139	381
Balance, end of period		473,967	897,563
Postretirement benefit plans [Member]
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year		(28,406)
Other comprehensive income (loss) before reclassifications		0
Amounts reclassified from accumulated other comprehensive loss		208	381
Net current-period other comprehensive income (loss)		208
Balance, end of period		(28,198)
Net unrealized loss on available-for-sale debt securities [Member]
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year		(275)
Other comprehensive income (loss) before reclassifications	[1]	189
Amounts reclassified from accumulated other comprehensive loss		0
Net current-period other comprehensive income (loss)		189
Balance, end of period		(86)
Unrealized gain on securities arising during the period, tax		66
Net unrealized loss on cash flow hedge [Member]
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year		(1,097)
Other comprehensive income (loss) before reclassifications	[2]	(4,980)
Amounts reclassified from accumulated other comprehensive loss		(69)	0
Net current-period other comprehensive income (loss)		(5,049)
Balance, end of period		(6,146)
Unrealized loss on cash flow hedge arising during the period, tax		(1,736)
Currency translation adjustment [Member]
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year		(18,169)
Other comprehensive income (loss) before reclassifications		(7,355)
Amounts reclassified from accumulated other comprehensive loss		0
Net current-period other comprehensive income (loss)		(7,355)
Balance, end of period		(25,524)
Accumulated other comprehensive loss [Member]
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year		(47,947)	(56,579)
Other comprehensive income (loss) before reclassifications		(12,146)
Amounts reclassified from accumulated other comprehensive loss		139
Net current-period other comprehensive income (loss)		(12,007)
Balance, end of period		$ (59,954)	$ (54,689)

[1] Other comprehensive income before reclassifications is net of income tax expense of $66.

[2] Other comprehensive loss before reclassifications is net of an income tax benefit of $1,736.