Other comprehensive income (accumulated other comprehensive loss) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year		$ 894,149	$ 1,075,136	$ 915,413	$ 1,015,013
Amounts reclassified from accumulated other comprehensive loss		321	319	1,082	650
Net current-period other comprehensive income (loss)				(605)
Balance, end of period		525,527	955,295	525,527	955,295
Postretirement benefit plans [Member]
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year				(36,529)
Other comprehensive income (loss) before reclassifications				0
Amounts reclassified from accumulated other comprehensive loss		381	319	1,142	650
Net current-period other comprehensive income (loss)				1,142
Balance, end of period		(35,387)		(35,387)
Net unrealized loss on available-for-sale debt securities [Member]
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year				(323)
Other comprehensive income (loss) before reclassifications	[1]			122
Amounts reclassified from accumulated other comprehensive loss				0
Net current-period other comprehensive income (loss)				122
Balance, end of period		(201)		(201)
Unrealized gain on securities arising during the period, tax				43
Net unrealized loss on cash flow hedge [Member]
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year				0
Other comprehensive income (loss) before reclassifications	[2]			(1,790)
Amounts reclassified from accumulated other comprehensive loss		(60)	0	(60)	0
Net current-period other comprehensive income (loss)				(1,850)
Balance, end of period		(1,850)		(1,850)
Unrealized loss on cash flow hedge arising during the period, tax				(635)
Currency translation adjustment [Member]
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year				(19,727)
Other comprehensive income (loss) before reclassifications				(19)
Amounts reclassified from accumulated other comprehensive loss				0
Net current-period other comprehensive income (loss)				(19)
Balance, end of period		(19,746)		(19,746)
Accumulated other comprehensive loss [Member]
Accumulated other comprehensive loss [Line Items]
Balance, beginning of year		(53,527)	(49,170)	(56,579)	(37,597)
Other comprehensive income (loss) before reclassifications				(1,687)
Amounts reclassified from accumulated other comprehensive loss				1,082
Balance, end of period		$ (57,184)	$ (48,989)	$ (57,184)	$ (48,989)

[1] Other comprehensive income before reclassifications is net of income tax expense of $43.

[2] Other comprehensive loss before reclassifications is net of an income tax benefit of $635.