Estimated net loss and prior service credit (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amortization of net gains (losses)	$ (7,100,000)
Defined benefit plan, amortization of net prior service cost (credit)	71,000
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amortization of net gains (losses)	(2,600,000)
Defined benefit plan, amortization of net prior service cost (credit)	$ (1,500,000)