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Income taxes - Tax Effect on Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Deferred tax assets: |  |
Non-capital loss, investment tax credits, currently non-deductible interest expenses, and financing costs | $ 329,099 | $ 328,679
Pension and OPEB | 48,586 | 43,638
Acquisition-related costs | 1,420 | 1,601
Environmental obligation | 14,790 | 14,803
Reserves and other non-deductible costs | 20,517 | 30,652
Regulatory liabilities | 161,560 | 154,597
Financial derivatives | 12,831 |
Other | 10,425 | 16,384
Total deferred income tax assets | 599,228 | 597,961
Less valuation allowance | (28,018) | (19,951)
Total deferred tax assets | 571,210 | 578,010
Deferred tax liabilities: |  |
Property, plant and equipment | (653,962) | (668,083)
Intangible assets | (7,247) | (7,157)
Outside basis in partnership | (167,659) | (125,519)
Regulatory accounts | (113,758) | (114,062)
Financial derivatives | 0 | (980)
Other | (314) | 0
Total deferred tax liabilities | (942,940) | (915,801)
Net deferred tax liabilities | (371,730) | (337,791)
Deferred tax assets | 72,415 | 61,357
Deferred tax liabilities | $ (444,145) | $ (399,148)

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- Definition Amount of deferred tax liability attributable to taxable temporary differences. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(15)(b)(2)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(15)(2)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(26)(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(26)(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319 + Details
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