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Pension and other postretirement benefits - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Pension Plans [Member]          
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]          
Service cost $ 1,224 $ 1,270 $ 4,651 $ 6,346 $ 5,605
Interest cost 3,332 3,480 13,892 13,388 11,629
Expected return on plan assets (4,749) (4,816) (18,774) (21,241) (14,694)
Amortization of prior service costs 27 28 106 110 77
Amortization of net loss 1,232 1,043 4,666 3,980 1,907
Settlements     0 0 4,701
Curtailment cost (benefit)     0 1,235 263
Net periodic benefit cost 1,066 1,005 4,541 3,818 9,488
Other Postretirement Benefit Plans [Member]          
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]          
Service cost 247 330 1,003 1,970 1,591
Interest cost 1,366 1,456 5,595 5,985 6,420
Expected return on plan assets     0 0 0
Amortization of prior service costs (338) (695) (2,781) (3,728) (3,844)
Amortization of net loss 911 876 3,537 3,814 3,704
Settlements     0 0 0
Curtailment cost (benefit)     0 (4,266) 0
Net periodic benefit cost $ 2,186 $ 1,967 $ 7,354 $ 3,775 $ 7,871