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Components of net periodic benefit cost | 6 Months Ended
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Jun. 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 June 30, |  | Six months ended June 30,
| 2013 | 2012 |  | 2013 | 2012
Service cost | $ | 926 |  | $ | 506 |  |  | $ | 1,606 |  | $ | 1,401 |
Interest cost | 1,755 |  | 1,606 |  |  | 3,390 |  | 3,436 |
Expected return on plan assets | (1,893 | ) | (1,782 | ) |  | (3,613 | ) | (3,482 | )
Amortization of prior service cost | 22 |  | 34 |  |  | 54 |  | 69 |
Amortization of net loss | 857 |  | 1,133 |  |  | 1,706 |  | 1,821 |
Curtailment | (18 | ) | — |  |  | (18 | ) | — |
Net periodic benefit cost | $ | 1,649 |  | $ | 1,497 |  |  | $ | 3,125 |  | $ | 3,245 |
| Other Postretirement Benefits ("OPEB")
| Three months ended June 30, |  | Six months ended June 30,
| 2013 | 2012 |  | 2013 | 2012
Service cost | $ | 670 |  | $ | 370 |  |  | $ | 1,163 |  | $ | 895 |
Interest cost | 1,433 |  | 1,271 |  |  | 2,756 |  | 2,757 |
Amortization of prior service cost | (938 | ) | (1,065 | ) |  | (1,998 | ) | (2,125 | )
Amortization of net loss | 1,273 |  | 1,808 |  |  | 2,776 |  | 3,304 |
Curtailment | (20 | ) | — |  |  | (20 | ) | — |
Net periodic benefit cost | $ | 2,418 |  | $ | 2,384 |  |  | $ | 4,677 |  | $ | 4,831 |
Employer contributions During the six months ended June 30, 2013, we have made contributions of approximately $7,419 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Implementation Guide (Q and A) -Number FAS88 -Paragraph 63 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5, 6, 7, 8 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Emerging Issues Task Force (EITF) -Number 03-2 -Paragraph 8 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. 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Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 106 -Paragraph 518 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 10: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph h -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. 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Reference 13: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 30 -Paragraph 26 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 14: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 8 -Subparagraph m -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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