CPG provides defined contribution plans and noncontributory defined benefit retirement plans that cover employees of subsidiaries of Columbia OpCo. Prior to the Separation, employees of subsidiaries of Columbia OpCo were covered by defined contribution plans and noncontributory defined benefit retirement plans provided by NiSource. Benefits under the defined benefit retirement plans reflect the employees' compensation, years of service and age at retirement. Additionally, CPG provides health care and life insurance benefits for certain retired employees of subsidiaries of Columbia OpCo. The majority of employees may become eligible for these benefits if they reach retirement age while working for subsidiaries of Columbia OpCo. The expected cost of such benefits is accrued during the employees' years of service. Current rates charged to customers of subsidiaries of Columbia OpCo include postretirement benefit costs. Cash contributions are remitted to trusts, including a grantor trust used to fund benefits of the CPG non-qualified defined benefit plan.

Subsidiaries of Columbia OpCo are participants in the consolidated CPG defined benefit retirement plans (the "Plans") and, therefore, subsidiaries of Columbia OpCo are allocated a ratable portion of CPG's trusts for the Plans in which its employees and retirees participate. As a result, the Partnership follows multiple employer accounting under the provisions of GAAP.

The following table provides the key assumptions that were used to calculate the pension and other postretirement benefit obligation and the net periodic benefit cost at the measurement dates of July 1, 2016 and December 31, 2015.