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v3.6.0.2

Components of deferred tax assets and liabilities (Details 3) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Deferred tax assets: |  |
Postretirement | $ 87,872 | $ 97,666
Compensation-related | 44,995 | 33,714
Alternative minimum tax credit carryforward | 29,338 | 28,169
Federal renewable energy credit | 16,944 | 3,400
Customer advances | 13,524 | 12,623
Legal and environmental contingencies | 9,895 | 6,377
Asset retirement obligations | 8,867 | 8,694
Other | 46,957 | 43,306
Total deferred tax assets | 258,392 | 233,949
Deferred tax liabilities: |  |
Depreciation and basis differences on property, plant and equipment | 774,838 | 756,444
Postretirement | 70,670 | 71,835
Intangible asset amortization | 26,413 | 23,950
Other | 45,580 | 36,359
Total deferred tax liabilities | 917,501 | 888,588
Valuation allowance | 9,117 | 8,990
Deferred Tax Liabilities, Net | $ 668,226 | $ 663,629

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