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Summary of Significant Accounting Policies - Other Expense (Income) - Net (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Accounting Policies [Abstract] |  |  |
Interest income | $ (1,952) | $ (2,587) | $ (2,710)
Interest on regulatory deferrals | (1,222) | (1,460) | (990)
Equity-related AFUDC | (6,970) | (6,585) | (6,554)
Non-service portion of pension and other postretirement benefit expenses | 6,433 | 8,899 | 5,156
Net (income) loss on investments | (905) | (14,299) | 5,369
Other expense (income) | (201) | 1,104 | 1,187
Total | $ (4,817) | $ (14,928) | $ 1,458

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- Definition Amount of realized and unrealized gain (loss) on investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27405-111563 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c),9(a)) -URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227 Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
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