Expected rate of return (Details 6)	12 Months Ended
	Dec. 31, 2012
Equity Securities [Member] | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target plan asset allocations range maximum	70.00%
Defined benefit plan, target plan asset allocations range minimum	60.00%
Equity Securities [Member] | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target plan asset allocations range maximum	75.00%
Defined benefit plan, target plan asset allocations range minimum	65.00%
Fixed Income Securities [Member] | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target plan asset allocations range maximum	40.00%
Defined benefit plan, target plan asset allocations range minimum	30.00%
Fixed Income Securities [Member] | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target plan asset allocations range maximum	35.00%
Defined benefit plan, target plan asset allocations range minimum	25.00%