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Estimated future benefit payments and subsidies (Details 10) - Underfunded Plan [Member] - Qualified Plan [Member] $ in Thousands | Dec. 31, 2017 USD ($)
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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

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