Estimated net loss and prior service credit (Details 4) - Qualified Plan [Member] - Underfunded Plan [Member]	Dec. 31, 2017 USD ($)
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)