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Components of net periodic benefit cost | 3 Months Ended
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Mar. 31, 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 March 31,
|  | 2016 | 2015
Service cost |  | $ | 1,270 |  | $ | 1,704 |
Interest cost |  | 3,480 |  | 3,388 |
Expected return on plan assets |  | (4,816 | ) | (5,494 | )
Amortization of prior service costs |  | 28 |  | 25 |
Amortization of net loss |  | 1,043 |  | 620 |
Net periodic benefit cost |  | $ | 1,005 |  | $ | 243 |
| Other Postretirement Benefits ("OPEB")
|  | Three months ended March 31,
|  | 2016 | 2015
Service cost |  | $ | 330 |  | $ | 562 |
Interest cost |  | 1,456 |  | 1,591 |
Amortization of prior service cost |  | (695 | ) | (961 | )
Amortization of net loss |  | 876 |  | 865 |
Net periodic benefit cost |  | $ | 1,967 |  | $ | 2,057 |
Employer contributions During the three months ended March 31, 2016, we made contributions of $464 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=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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