Income tax Effective Income Tax Rate Reconcilitation (Details)	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	35.00%
Federal tax credits	(18.00%)	(16.10%)
State and local taxes, net of federal tax benefit	6.50%	5.00%
Flow through depreciation and cost basis differences	1.00%	1.00%
Other	0.60%	(0.90%)
Effective tax rate	11.10%	24.00%

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(2))
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-Publisher FASB
-Name Accounting Standards Codification
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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to depreciation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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