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Components of deferred tax assets and liabilities (Details 3) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Deferred tax assets:    
Postretirement $ 97,666 $ 99,853
Compensation-related 33,844 35,669
Alternative minimum tax credit carryforward 28,173 23,678
Customer advances 12,623 12,245
Asset retirement obligations 8,694 7,894
Legal and environmental contingencies 6,377 7,890
Other 58,202 52,862
Total deferred tax assets 245,579 240,091
Deferred tax liabilities:    
Depreciation and basis differences on property, plant and equipment 791,368 773,160
Postretirement 71,835 70,642
Intangible asset amortization 23,950 22,810
Other 36,906 46,637
Total deferred tax liabilities 924,059 913,249
Valuation allowance 8,990 8,852
Deferred Tax Liabilities, Net $ (687,470) $ (682,010)