Income tax Effective Income Tax Rate Reconcilitation (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	35.00%	21.00%	35.00%
Federal tax credits	(12.30%)	(12.80%)	(15.80%)	(14.40%)
State and local taxes, net of federal tax benefit	6.50%	5.30%	6.50%	5.20%
Flow through depreciation and cost basis differences	(0.10%)	3.40%	(2.30%)	1.00%
Other	(0.60%)	(6.40%)	3.00%	(2.70%)
Effective tax rate	14.50%	24.50%	12.40%	24.10%