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Components of net periodic benefit cost | 6 Months Ended
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Jun. 30, 2017
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,
| 2017 |  | 2016 |  | 2017 |  | 2016
Service cost | $ | 968 |  |  | $ | 1,270 |  |  | $ | 2,192 |  |  | $ | 2,540 |
Interest cost | 3,323 |  |  | 3,482 |  |  | 6,655 |  |  | 6,962 |
Expected return on plan assets | (5,116 | ) |  | (4,816 | ) |  | (9,865 | ) |  | (9,632 | )
Amortization of prior service costs | 26 |  |  | 28 |  |  | 53 |  |  | 56 |
Amortization of net loss | 1,506 |  |  | 1,041 |  |  | 2,738 |  |  | 2,084 |
Net periodic benefit cost | $ | 707 |  |  | $ | 1,005 |  |  | $ | 1,773 |  |  | $ | 2,010 |
| Other Postretirement Benefits ("OPEB")
| Three months ended June 30, |  | Six months ended June 30,
| 2017 |  | 2016 |  | 2017 |  | 2016
Service cost | $ | 191 |  |  | $ | 330 |  |  | $ | 438 |  |  | $ | 660 |
Interest cost | 2,978 |  |  | 1,439 |  |  | 4,344 |  |  | 2,895 |
Amortization of prior service cost | (698 | ) |  | (695 | ) |  | (1,036 | ) |  | (1,390 | )
Amortization of net loss | (929 | ) |  | 879 |  |  | (18 | ) |  | 1,755 |
Net periodic benefit cost | $ | 1,542 |  |  | $ | 1,953 |  |  | $ | 3,728 |  |  | $ | 3,920 |
Employer contributions During the six months ended June 30, 2017, we made contributions of $874 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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