Components of deferred tax assets and liabilities (Details 4) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Postretirement	$ 51,075	$ 51,930
Compensation-related	37,330	29,885
Operating lease liabilities	24,459
Asset retirement obligations	7,450	7,083
Customer advances	7,325	7,734
Legal and environmental contingencies	6,601	6,729
Federal renewable energy credit	5,343	8,015
Alternative minimum tax credit carryforward	0	13,404
Other	32,533	37,347
Total deferred tax assets	172,116	162,127
Deferred tax liabilities:
Depreciation and basis differences on property, plant and equipment	511,867	476,832
Postretirement	48,927	44,432
Operating lease right-of-use-assets	24,436
Intangible asset amortization	18,930	17,752
Other	61,385	39,712
Total deferred tax liabilities	665,545	578,728
Valuation allowance	13,154	13,484
Net deferred income tax liability	$ 506,583	$ 430,085