Pension and other postretirement benefits - Change in benefit obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Pension Plans [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	$ 326,571	$ 336,292
Service cost	4,651	6,346	$ 5,605
Interest cost	13,892	13,388	11,629
Plan amendments	0	0
Actuarial loss (gain)	12,761	(11,429)
Medicare Part D	0	0
Benefits paid	(29,828)	(19,247)
Curtailment	0	1,221
Benefit obligation at end of year	328,047	326,571	336,292
Other Postretirement Benefit Plans [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	132,550	158,781
Service cost	1,003	1,970	1,591
Interest cost	5,595	5,985	6,420
Plan amendments	0	(1,758)
Actuarial loss (gain)	1,919	(18,150)
Medicare Part D	38	0
Benefits paid	(7,249)	(5,878)
Curtailment	0	(8,400)
Benefit obligation at end of year	$ 133,856	$ 132,550	$ 158,781