Change in Projected Benefit Obligation | Pension Benefits | Postretirement Health Care Benefits | ||||||||||||||||||
Qualified Plans | Nonqualified Plans | |||||||||||||||||||
(millions) | 2013 | 2012 | 2013 | 2012 | 2013 | 2012 | ||||||||||||||
Benefit obligation at beginning of period | $ | 3,164 | $ | 3,015 | $ | 37 | $ | 38 | $ | 121 | $ | 100 | ||||||||
Service cost | 117 | 120 | 1 | 1 | 6 | 10 | ||||||||||||||
Interest cost | 136 | 137 | 1 | 2 | 2 | 3 | ||||||||||||||
Actuarial (gain)/loss | (125 | ) | 107 | — | — | (3 | ) | 18 | ||||||||||||
Participant contributions | 1 | 1 | — | — | 5 | 5 | ||||||||||||||
Benefits paid | (122 | ) | (126 | ) | (4 | ) | (3 | ) | (14 | ) | (12 | ) | ||||||||
Plan amendments | 2 | (90 | ) | — | (1 | ) | (44 | ) | (3 | ) | ||||||||||
Benefit obligation at end of period | $ | 3,173 | $ | 3,164 | $ | 35 | $ | 37 | $ | 73 | $ | 121 | ||||||||