Estimated future benefit payments and subsidies (Details 9) $ in Thousands	Dec. 31, 2016 USD ($)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next twelve months	$ 5,410
Prescription drug subsidy receipts, next twelve months	168
Expected future benefit payments, year two	5,573
Prescription drug subsidy receipts, year two	165
Expected future benefit payments, year three	5,603
Prescription drug subsidy receipts, year three	160
Expected future benefit payments, year four	5,500
Prescription drug subsidy receipts, Year Four	154
Expected future benefit payments, year five	5,511
Prescription drug subsidy receipts, year five	146
Expected future benefit payments, five fiscal years thereafter	27,956
Prescription drug subsidy receipts, after year five	568
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next twelve months	24,798
Expected future benefit payments, year two	25,054
Expected future benefit payments, year three	25,271
Expected future benefit payments, year four	25,616
Expected future benefit payments, year five	25,987
Expected future benefit payments, five fiscal years thereafter	$ 132,224