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