Income tax Effective Income Tax Rate Reconcilitation (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	21.00%	21.00%
Federal tax credits	(11.20%)	14.80%	(10.10%)	(23.60%)
State and local taxes, net of federal tax benefit	8.90%	4.20%	8.70%	10.40%
Flow through depreciation and cost basis differences	(1.50%)	17.50%	(1.00%)	(7.20%)
Excess deferred tax amortization	(4.60%)	(0.40%)	(3.70%)	(2.80%)
Other	0.00%	0.00%	(4.20%)	0.00%
Effective tax rate	9.10%	61.40%	9.20%	(4.00%)
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent	(3.50%)	4.30%	(1.50%)	(1.80%)