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