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Components of deferred tax assets and liabilities (Details 4) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Deferred tax assets: |  |
Postretirement | $ 55,736 | $ 87,872
Compensation-related | 16,298 | 44,995
Alternative minimum tax credit carryforward | 37,683 | 29,338
Federal renewable energy credit | 19,367 | 16,944
Customer advances | 8,712 | 13,524
Legal and environmental contingencies | 7,363 | 9,895
Asset retirement obligations | 6,380 | 8,867
Other | 35,738 | 46,957
Total deferred tax assets | 187,277 | 258,392
Deferred tax liabilities: |  |
Depreciation and basis differences on property, plant and equipment | 429,577 | 774,838
Postretirement | 43,505 | 70,670
Intangible asset amortization | 16,979 | 26,413
Other | 32,591 | 45,580
Total deferred tax liabilities | 522,652 | 917,501
Valuation allowance | 11,896 | 9,117
Deferred Tax Liabilities, Net | $ 347,271 | $ 668,226

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