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Components of net periodic benefit cost | 6 Months Ended
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Jun. 30, 2015
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,
| 2015 | 2014 |  | 2015 | 2014
Service cost | $ | 1,509 |  | $ | 1,079 |  |  | $ | 3,213 |  | $ | 2,666 |
Interest cost | 3,267 |  | 3,101 |  |  | 6,655 |  | 5,741 |
Expected return on plan assets | (5,269 | ) | (3,701 | ) |  | (10,763 | ) | (7,098 | )
Amortization of prior service costs | 30 |  | 9 |  |  | 55 |  | 18 |
Amortization of net loss | 1,405 |  | 167 |  |  | 2,024 |  | 953 |
Curtailment | (12 | ) | — |  |  | (12 | ) | — |
Net periodic benefit cost | $ | 930 |  | $ | 655 |  |  | $ | 1,172 |  | $ | 2,280 |
| Other Postretirement Benefits ("OPEB")
| Three months ended June 30, |  | Six months ended June 30,
| 2015 | 2014 |  | 2015 | 2014
Service cost | $ | 487 |  | $ | 117 |  |  | $ | 1,049 |  | $ | 794 |
Interest cost | 1,390 |  | 1,695 |  |  | 2,981 |  | 3,206 |
Amortization of prior service cost | (961 | ) | (961 | ) |  | (1,922 | ) | (1,922 | )
Amortization of net loss | 1,234 |  | 828 |  |  | 2,099 |  | 1,852 |
Net periodic benefit cost | $ | 2,150 |  | $ | 1,679 |  |  | $ | 4,207 |  | $ | 3,930 |
Employer contributions During the six months ended June 30, 2015, we made contributions of approximately $2,673 to the qualified defined benefit plans we sponsor.

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- Definition Components of net periodic benefit cost [Abstract] + References No definition available. + Details
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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 -SubTopic 80 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=49170332&loc=SL14450702-114947 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e2410-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39675-114964 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414199&loc=d3e39622-114963 + Details
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