Income tax Effective Income Tax Rate Reconcilitation (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	21.00%	21.00%
Federal tax credits	(15.10%)	(17.00%)	(13.30%)	(17.50%)
State and local taxes, net of federal tax benefit	6.50%	6.50%	6.50%	6.50%
Flow through depreciation and cost basis differences	0.40%	(2.20%)	1.40%	(3.40%)
Excess deferred tax amortization	(2.70%)	0.00%	(3.20%)	0.00%
Other	0.60%	3.20%	0.10%	4.70%
Effective tax rate	10.70%	11.50%	12.50%	11.30%