Income tax Effective Income Tax Rate Reconcilitation (Details)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%
Federal tax credits	(12.70%)	(18.00%)
State and local taxes, net of federal tax benefit	6.50%	6.50%
Flow through depreciation and cost basis differences	1.30%	1.00%
Amortization of Deferred Charges	(3.70%)
Other	0.70%	0.60%
Effective tax rate	13.10%	11.10%