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Components of net periodic benefit cost | 9 Months Ended
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Sep. 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 September 30, |  | Nine months ended September 30,
| 2016 | 2015 |  | 2016 | 2015
Service cost | $ | 1,270 |  | $ | 2,074 |  |  | $ | 3,810 |  | $ | 5,287 |
Interest cost | 3,478 |  | 3,348 |  |  | 10,440 |  | 10,003 |
Expected return on plan assets | (4,813 | ) | (5,262 | ) |  | (14,445 | ) | (16,025 | )
Amortization of prior service costs | 28 |  | 28 |  |  | 84 |  | 83 |
Amortization of net loss | 1,041 |  | 1,025 |  |  | 3,125 |  | 3,049 |
Curtailment | — |  | 377 |  |  | — |  | 365 |
Net periodic benefit cost | $ | 1,004 |  | $ | 1,590 |  |  | $ | 3,014 |  | $ | 2,762 |
| Other Postretirement Benefits ("OPEB")
| Three months ended September 30, |  | Nine months ended September 30,
| 2016 | 2015 |  | 2016 | 2015
Service cost | $ | 330 |  | $ | 468 |  |  | $ | 990 |  | $ | 1,517 |
Interest cost | 1,469 |  | 1,542 |  |  | 4,364 |  | 4,523 |
Amortization of prior service cost | (695 | ) | (1,007 | ) |  | (2,085 | ) | (2,929 | )
Amortization of net loss | 878 |  | 874 |  |  | 2,633 |  | 2,973 |
Net periodic benefit cost | $ | 1,982 |  | $ | 1,877 |  |  | $ | 5,902 |  | $ | 6,084 |
Employer contributions During the nine months ended September 30, 2016, we made contributions of $1,338 to the qualified defined benefit and unqualified supplemental executive retirement benefit ("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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