Table:
|  |  | Years Ended June 30, |  |
| --- | --- | --- | --- |
| ($ in millions) | 2019 | 2018 | 2017 |
| Pension plans | $9.8 | $11.3 | $45.8 |
| Other postretirement plans | 1.8 | 2.9 | 2.6 |
| Net periodic benefit costs | $11.6 | $14.2 | $48.4 |
Paragraphs:
Net Pension Expense
Net pension expense, as we define it below, includes the net periodic benefit costs related to both our pension and other postretirement plans. The net periodic benefit costs are determined annually based on beginning of year balances and are recorded ratably throughout the fiscal year, unless a significant re-measurement event occurs. The following is a summary of the net periodic benefit costs for the years ended June 30, 2019, 2018 and 2017:
In September 2016, we announced changes to retirement plans we offer to certain employees. Benefits accrued to eligible participants of our largest qualified defined benefit pension plan and certain non-qualified pension plans were frozen effective December 31, 2016. Approximately 1,900 affected employees were transitioned to the Company’s 401(k) plan that has been in effect for eligible employees since 2012, when the pension plan was closed to new entrants. We recognized the plan freeze during fiscal year 2017 as a curtailment, since it eliminated the accrual for a significant number of participants for all of their future services. We also made a voluntary pension contribution of $100.0 million to the affected plan in October 2016.
The service cost component of net pension expense represents the estimated cost of future pension liabilities earned associated with active employees. The pension earnings, interest and deferrals (“pension EID”) is comprised of the expected return on plan assets, interest costs on the projected benefit obligations of the plans and amortization of actuarial gains and losses and prior service costs.
Question:
When was the changes to retirement plans offered to certain employees announced?
Answer: In September 2016