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Components of deferred tax assets and liabilities (Details 4) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Deferred tax assets: |  |
Postretirement | $ 51,930 | $ 55,736
Compensation-related | 29,885 | 16,298
Alternative minimum tax credit carryforward | 13,404 | 37,683
Federal renewable energy credit | 8,015 | 19,367
Customer advances | 7,734 | 8,712
Asset retirement obligations | 7,083 | 6,380
Legal and environmental contingencies | 6,729 | 7,363
Other | 37,347 | 35,738
Total deferred tax assets | 162,127 | 187,277
Deferred tax liabilities: |  |
Depreciation and basis differences on property, plant and equipment | 476,832 | 429,577
Postretirement | 44,432 | 43,505
Intangible asset amortization | 17,752 | 16,979
Other | 39,712 | 32,591
Total deferred tax liabilities | 578,728 | 522,652
Valuation allowance | 13,484 | 11,896
Deferred Tax Liabilities, Net | $ 430,085 | $ 347,271

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