Pension and other postretirement benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 4,384	$ 4,651	$ 6,346
Interest cost	13,310	13,892	13,388
Expected return on plan assets	(18,997)	(18,774)	(21,241)
Amortization of prior service costs	106	106	110
Amortization of net loss	4,745	4,666	3,980
Curtailment cost (benefit)	0	0	1,235
Net periodic benefit cost	3,548	4,541	3,818
Other Postretirement Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	801	1,003	1,970
Interest cost	5,317	5,595	5,985
Expected return on plan assets	0	0	0
Amortization of prior service costs	(3,692)	(2,781)	(3,728)
Amortization of net loss	3,865	3,537	3,814
Curtailment cost (benefit)	(1,364)	0	(4,266)
Net periodic benefit cost	$ 4,927	$ 7,354	$ 3,775