Income tax Effective Income Tax Rate Reconcilitation (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	21.00%	21.00%
Federal tax credits	14.80%	(14.80%)	(23.60%)	(13.80%)
State and local taxes, net of federal tax benefit	4.20%	6.50%	10.40%	6.50%
Flow through depreciation and cost basis differences	17.50%	1.00%	(7.20%)	1.20%
Excess deferred tax amortization	(0.40%)	(3.90%)	(2.80%)	(3.50%)
Other	4.30%	0.00%	(1.80%)	0.20%
Effective tax rate	61.40%	9.80%	(4.00%)	11.60%