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Income tax Effective Income Tax Rate Reconcilitation (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Income Tax Disclosure [Abstract]        
Federal statutory tax rate 21.00% 35.00% 21.00% 35.00%
Federal tax credits (17.00%) (12.40%) (17.50%) (15.00%)
State and local taxes, net of federal tax benefit 6.50% 5.20% 6.50% 5.10%
Flow through depreciation and cost basis differences (2.20%) (1.70%) (3.40%) 0.10%
Other 3.20% (2.30%) 4.70% (1.30%)
Effective tax rate 11.50% 23.80% 11.30% 23.90%