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Components of net periodic benefit cost | 6 Months Ended
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Jun. 30, 2016
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,
| 2016 | 2015 |  | 2016 | 2015
Service cost | $ | 1,270 |  | $ | 1,509 |  |  | $ | 2,540 |  | $ | 3,213 |
Interest cost | 3,482 |  | 3,267 |  |  | 6,962 |  | 6,655 |
Expected return on plan assets | (4,816 | ) | (5,269 | ) |  | (9,632 | ) | (10,763 | )
Amortization of prior service costs | 28 |  | 30 |  |  | 56 |  | 55 |
Amortization of net loss | 1,041 |  | 1,405 |  |  | 2,084 |  | 2,024 |
Curtailment | — |  | (12 | ) |  | — |  | (12 | )
Net periodic benefit cost | $ | 1,005 |  | $ | 930 |  |  | $ | 2,010 |  | $ | 1,172 |
| Other Postretirement Benefits ("OPEB")
| Three months ended June 30, |  | Six months ended June 30,
| 2016 | 2015 |  | 2016 | 2015
Service cost | $ | 330 |  | $ | 487 |  |  | $ | 660 |  | $ | 1,049 |
Interest cost | 1,439 |  | 1,390 |  |  | 2,895 |  | 2,981 |
Amortization of prior service cost | (695 | ) | (961 | ) |  | (1,390 | ) | (1,922 | )
Amortization of net loss | 879 |  | 1,234 |  |  | 1,755 |  | 2,099 |
Net periodic benefit cost | $ | 1,953 |  | $ | 2,150 |  |  | $ | 3,920 |  | $ | 4,207 |
Employer contributions During the six months ended June 30, 2016, we made contributions of $901 to the qualified defined benefit and unqualified supplemental executive retirement benefits ("SERB") plans that 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=65877416&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=65897934&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=66047640&loc=d3e39622-114963 + Details
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