Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,096	$ 1,270	$ 3,288	$ 3,810
Interest cost	3,327	3,478	9,982	10,440
Expected return on plan assets	(4,750)	(4,813)	(14,615)	(14,445)
Amortization of prior service costs	27	28	80	84
Amortization of net loss	1,187	1,041	3,925	3,125
Net periodic benefit cost	887	1,004	2,660	3,014
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	219	330	657	990
Interest cost	1,382	1,469	5,726	4,364
Amortization of prior service costs	(695)	(695)	(1,731)	(2,085)
Amortization of net loss	958	878	940	2,633
Net periodic benefit cost	$ 1,864	$ 1,982	$ 5,592	$ 5,902