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