Income tax Effective Income Tax Rate Reconcilitation (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	21.00%	21.00%
Federal tax credits	(14.40%)	(15.10%)	(12.00%)	(13.30%)
State and local taxes, net of federal tax benefit	10.90%	6.50%	8.50%	6.50%
Flow through depreciation and cost basis differences	(2.60%)	0.40%	0.30%	1.40%
Excess deferred tax amortization	(2.50%)	(2.70%)	(2.10%)	(3.20%)
Other	(1.00%)	0.60%	0.40%	0.10%
Effective tax rate	11.40%	10.70%	16.10%	12.50%