Components of deferred tax assets and liabilities (Details 3) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Postretirement	$ 51,495	$ 51,075
Compensation-related	40,477	37,330
Operating lease liabilities	25,963	24,459
Payroll tax deferral	14,010	0
Legal and environmental contingencies	9,467	6,601
Asset retirement obligations	8,060	7,450
Customer advances	7,463	7,325
Federal renewable energy credit	0	5,343
Other	37,944	32,533
Total deferred tax assets	194,879	172,116
Deferred tax liabilities:
Depreciation and basis differences on property, plant and equipment	536,966	511,867
Postretirement	49,233	48,927
Operating lease right-of-use-assets	25,858	24,436
Intangible asset amortization	19,514	18,930
Other	67,922	61,385
Total deferred tax liabilities	699,493	665,545
Valuation allowance	11,484	13,154
Net deferred income tax liability	$ 516,098	$ 506,583