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Components of net periodic benefit cost | 9 Months Ended
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Sep. 30, 2013
Components of net periodic benefit cost [Abstract] |
Components of net periodic benefit cost | Components of net periodic benefit cost
| Pension Benefits
| Three months ended September 30, |  | Nine months ended September 30,
| 2013 | 2012 |  | 2013 | 2012
Service cost | $ | 1,686 |  | $ | 701 |  |  | $ | 3,292 |  | $ | 2,102 |
Interest cost | 2,903 |  | 1,717 |  |  | 6,293 |  | 5,153 |
Expected return on plan assets | (3,723 | ) | (1,740 | ) |  | (7,337 | ) | (5,222 | )
Amortization of prior service cost | 30 |  | 34 |  |  | 85 |  | 103 |
Amortization of net loss | 723 |  | 910 |  |  | 2,428 |  | 2,731 |
Curtailment | — |  | — |  |  | (18 | ) | — |
Net periodic benefit cost | $ | 1,619 |  | $ | 1,622 |  |  | $ | 4,743 |  | $ | 4,867 |
| Other Postretirement Benefits ("OPEB")
| Three months ended September 30, |  | Nine months ended September 30,
| 2013 | 2012 |  | 2013 | 2012
Service cost | $ | 700 |  | $ | 448 |  |  | $ | 1,862 |  | $ | 1,343 |
Interest cost | 1,474 |  | 1,378 |  |  | 4,230 |  | 4,135 |
Amortization of prior service cost | (998 | ) | (1,063 | ) |  | (2,997 | ) | (3,188 | )
Amortization of net loss | 1,157 |  | 1,652 |  |  | 3,934 |  | 4,956 |
Curtailment | — |  | — |  |  | (20 | ) | — |
Net periodic benefit cost | $ | 2,333 |  | $ | 2,415 |  |  | $ | 7,009 |  | $ | 7,246 |
Employer contributions During the nine months ended September 30, 2013, we have made contributions of approximately $8,582 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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