Employee Benefits Allocation of Plan Assets (Details)	Dec. 31, 2017	Dec. 31, 2016
Allocation of Plan Assets
Defined benefit plan, plan assets, target allocation, percentage	100.00%
Defined benefit plan, actual plan asset allocations	100.00%	100.00%
Alternative Investments
Allocation of Plan Assets
Defined benefit plan, plan assets, target allocation, percentage	27.00%
Defined benefit plan, actual plan asset allocations	27.00%	21.00%
Equity securities
Allocation of Plan Assets
Defined benefit plan, plan assets, target allocation, percentage	23.00%
Defined benefit plan, actual plan asset allocations	23.00%	23.00%
Fixed income securities
Allocation of Plan Assets
Defined benefit plan, plan assets, target allocation, percentage	21.00%
Defined benefit plan, actual plan asset allocations	21.00%	26.00%
Liability driven investments
Allocation of Plan Assets
Defined benefit plan, plan assets, target allocation, percentage	14.00%
Defined benefit plan, actual plan asset allocations	14.00%	14.00%
Real estate
Allocation of Plan Assets
Defined benefit plan, plan assets, target allocation, percentage	6.00%
Defined benefit plan, actual plan asset allocations	6.00%	6.00%
Other plan assets
Allocation of Plan Assets
Defined benefit plan, plan assets, target allocation, percentage	9.00%
Defined benefit plan, actual plan asset allocations	9.00%	10.00%