Income tax Effective Income Tax Rate Reconcilitation (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	21.00%	21.00%
Federal tax credits	(14.20%)	(9.20%)	(11.10%)	(9.70%)
State and local taxes, net of federal tax benefit	5.80%	8.00%	7.90%	8.70%
Flow through depreciation and cost basis differences	(0.70%)	1.60%	0.10%	0.90%
Excess deferred tax amortization	(4.40%)	(4.20%)	(3.90%)	(4.30%)
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent	3.80%	(0.10%)	1.80%	(0.10%)
Effective tax rate	11.30%	17.10%	15.80%	16.50%