Income tax Effective Income Tax Rate Reconcilitation (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	21.00%	21.00%
Federal tax credits	(14.80%)	(12.30%)	(13.80%)	(15.80%)
State and local taxes, net of federal tax benefit	6.50%	6.50%	6.50%	6.50%
Flow through depreciation and cost basis differences	1.00%	(0.10%)	1.20%	(2.30%)
Excess deferred tax amortization	(3.90%)	0.00%	(3.50%)	0.00%
Other	0.00%	(0.60%)	0.20%	3.00%
Effective tax rate	9.80%	14.50%	11.60%	12.40%