Expected rate of return (Details 6) - Qualified Plan	Dec. 31, 2019
Underfunded Plan | Minimum | Equity securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	40.00%
Underfunded Plan | Minimum | Fixed Income Securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	50.00%
Underfunded Plan | Maximum | Equity securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	50.00%
Underfunded Plan | Maximum | Fixed Income Securities | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	60.00%
Overfunded Plan | Equity securities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	30.00%
Overfunded Plan | Fixed Income Securities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, target allocation, percentage	70.00%