Estimated future benefit payments and subsidies (Details 10) - Underfunded Plan [Member] - Qualified Plan [Member] $ in Thousands	Dec. 31, 2017 USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next twelve months	$ 25,111
Expected future benefit payments, year two	25,280
Expected future benefit payments, year three	25,587
Expected future benefit payments, year four	25,866
Expected future benefit payments, year five	26,185
Expected future benefit payments, five fiscal years thereafter	130,994
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next twelve months	5,490
Prescription drug subsidy receipts, next twelve months	152
Expected future benefit payments, year two	5,525
Prescription drug subsidy receipts, year two	147
Expected future benefit payments, year three	5,396
Prescription drug subsidy receipts, year three	141
Expected future benefit payments, year four	5,391
Prescription drug subsidy receipts, Year Four	132
Expected future benefit payments, year five	5,470
Prescription drug subsidy receipts, year five	123
Expected future benefit payments, five fiscal years thereafter	27,106
Prescription drug subsidy receipts, after year five	$ 454