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Components of net periodic benefit cost (Tables) | 9 Months Ended
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Sep. 30, 2013
Components of net periodic benefit cost [Abstract] |
Schedule of Net Benefit Costs |
| 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 |

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- Definition Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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