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Components of net periodic benefit cost | 6 Months Ended
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Jun. 30, 2014
Components of net periodic benefit cost [Abstract] |
Components of net periodic benefit cost | Components of net periodic benefit cost
| Pension Benefits
| Three months ended June 30, |  | Six months ended June 30,
| 2014 | 2013 |  | 2014 | 2013
Service cost | $ | 1,079 |  | $ | 926 |  |  | $ | 2,666 |  | $ | 1,606 |
Interest cost | 3,101 |  | 1,755 |  |  | 5,741 |  | 3,390 |
Expected return on plan assets | (3,701 | ) | (1,893 | ) |  | (7,098 | ) | (3,613 | )
Amortization of prior service costs | 9 |  | 22 |  |  | 18 |  | 54 |
Amortization of net loss | 167 |  | 857 |  |  | 953 |  | 1,706 |
Curtailment | — |  | (18 | ) |  | — |  | (18 | )
Net periodic benefit cost | $ | 655 |  | $ | 1,649 |  |  | $ | 2,280 |  | $ | 3,125 |
| Other Postretirement Benefits ("OPEB")
| Three months ended June 30, |  | Six months ended June 30,
| 2014 | 2013 |  | 2014 | 2013
Service cost | $ | 117 |  | $ | 670 |  |  | $ | 794 |  | $ | 1,163 |
Interest cost | 1,695 |  | 1,433 |  |  | 3,206 |  | 2,756 |
Amortization of prior service cost | (961 | ) | (938 | ) |  | (1,922 | ) | (1,998 | )
Amortization of net loss | 828 |  | 1,273 |  |  | 1,852 |  | 2,776 |
Curtailment | — |  | (20 | ) |  | — |  | (20 | )
Net periodic benefit cost | $ | 1,679 |  | $ | 2,418 |  |  | $ | 3,930 |  | $ | 4,677 |
Employer contributions During the six months ended June 30, 2014, we made contributions of approximately $1,413 to the qualified defined benefit plans we sponsor.

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- Definition The entire disclosure for pension and other postretirement benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -URI http://asc.fasb.org/topic&trid=2235017 + Details
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