Income tax Effective Income Tax Rate Reconcilitation (Details)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%
Federal tax credits	(10.90%)	(12.70%)
State and local taxes, net of federal tax benefit	7.40%	6.50%
Flow through depreciation and cost basis differences	1.60%	1.30%
Excess deferred tax amortization	(1.90%)	(3.70%)
Other	1.00%	0.70%
Effective tax rate	18.20%	13.10%