| | | | Three months ended March 31, |
| | | | Qualified and Non-qualified | | | | | | |
| | Pension Benefits | Postretirement |
| | | | United States | | International | | Benefits |
| (Millions) | | 2015 | | 2014 | | 2015 | | 2014 | | 2015 | | 2014 |
| Net periodic benefit cost (benefit) | | | | | | | | | | | | |
| Service cost | | $ | 73 | | $ | 60 | | $ | 42 | | $ | 36 | | $ | 21 | | $ | 16 |
| Interest cost | | | 164 | | | 169 | | | 55 | | | 64 | | | 25 | | | 24 |
| Expected return on plan assets | | | (267) | | | (261) | | | (81) | | | (79) | | | (22) | | | (22) |
| Amortization of transition (asset) obligation | | | ― | | | ― | | | ― | | | ― | | | ― | | | ― |
| Amortization of prior service cost (benefit) | | | (6) | | | 1 | | | (4) | | | (4) | | | (8) | | | (12) |
| Amortization of net actuarial (gain) loss | | | 102 | | | 61 | | | 38 | | | 31 | | | 19 | | | 14 |
| Net periodic benefit cost (benefit) | | $ | 66 | | $ | 30 | | $ | 50 | | $ | 48 | | $ | 35 | | $ | 20 |
| Settlements, curtailments, special termination benefits and other | | | ― | | | ― | | | (17) | | | ― | | | ― | | | ― |
| Net periodic benefit cost (benefit) after settlements, curtailments, | | | | | | | | | | | | | | | | | | |
| | special termination benefits and other | | $ | 66 | | $ | 30 | | $ | 33 | | $ | 48 | | $ | 35 | | $ | 20 |