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Estimated net loss and prior service credit (Details 4) - Qualified Plan [Member] - Underfunded Plan [Member] | Dec. 31, 2017 USD ($)
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Pension Benefits |
Defined Benefit Plan Disclosure [Line Items] |
Defined benefit plan, expected amortization of gain (loss), next year | $ (7,000,000.0)
Other Postretirement Benefits |
Defined Benefit Plan Disclosure [Line Items] |
Defined benefit plan, expected amortization of gain (loss), next year | (800,000)
Defined benefit plan, amortization of net prior service cost (credit) | $ (1,400,000)

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