Estimated net loss and prior service credit (Details 4)	12 Months Ended
	Dec. 31, 2016 USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amortization of net gains (losses)	$ (6,400,000)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amortization of net gains (losses)	(900,000)
Defined benefit plan, amortization of net prior service cost (credit)	$ (1,400,000)