The components of net periodic benefit cost for our pension plans were as follows (dollars in millions): | | | | | | | | | | | | | | | | | | Pension Plans | | Three Months Ended September 30 | | Nine Months Ended September 30 | | 2016 | | 2015 | | 2016 | | 2015 | Service cost | $ | 6.2 |
| | $ | 6.1 |
| | $ | 18.4 |
| | $ | 17.9 |
| Interest cost | 10.2 |
| | 11.6 |
| | 30.6 |
| | 34.6 |
| Expected return on plan assets | (12.3 | ) | | (13.3 | ) | | (37.1 | ) | | (39.8 | ) | Net amortization of unrecognized amounts | | | | | | | | Prior service cost | 1.4 |
| | 1.4 |
| | 4.3 |
| | 4.2 |
| Actuarial loss | 1.5 |
| | 2.1 |
| | 4.3 |
| | 6.4 |
| Net periodic benefit cost | $ | 7.0 |
| | $ | 7.9 |
| | $ | 20.5 |
| | $ | 23.3 |
|
The components of net periodic benefit cost for our postretirement plans were as follows (dollars in millions): | | | | | | | | | | | | | | | | | | Postretirement Plans | | Three Months Ended September 30 | | Nine Months Ended September 30 | | 2016 | | 2015 | | 2016 | | 2015 | Service cost | $ | 0.1 |
| | $ | 0.4 |
| | $ | 0.5 |
| | $ | 1.3 |
| Interest cost | 0.1 |
| | 0.3 |
| | 0.5 |
| | 0.9 |
| Net amortization of unrecognized amounts | | | | | | | | Prior service cost | (0.1 | ) | | — |
| | (0.1 | ) | | — |
| Actuarial (income) loss | — |
| | — |
| | (0.4 | ) | | 0.1 |
| Net periodic benefit cost | $ | 0.1 |
| | $ | 0.7 |
| | $ | 0.5 |
| | $ | 2.3 |
|
|