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Components of deferred tax assets and liabilities (Details 3) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Deferred tax assets: |  |
Postretirement | $ 51,495 | $ 51,075
Compensation-related | 40,477 | 37,330
Operating lease liabilities | 25,963 | 24,459
Payroll tax deferral | 14,010 | 0
Legal and environmental contingencies | 9,467 | 6,601
Asset retirement obligations | 8,060 | 7,450
Customer advances | 7,463 | 7,325
Federal renewable energy credit | 0 | 5,343
Other | 37,944 | 32,533
Total deferred tax assets | 194,879 | 172,116
Deferred tax liabilities: |  |
Depreciation and basis differences on property, plant and equipment | 536,966 | 511,867
Postretirement | 49,233 | 48,927
Operating lease right-of-use-assets | 25,858 | 24,436
Intangible asset amortization | 19,514 | 18,930
Other | 67,922 | 61,385
Total deferred tax liabilities | 699,493 | 665,545
Valuation allowance | 11,484 | 13,154
Net deferred income tax liability | $ 516,098 | $ 506,583

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