General synonyms:
* Unbiased
* Fair
* Objective
* Neutral
* Just
* Equitable
* Disinterested
* Nonpartisan
* Unprejudiced
More specific synonyms:
* Impartiality: This is the noun form of impartial.
* Equity: This emphasizes fairness and justice.
* Objectivity: This highlights the absence of personal bias.
* Detachment: This emphasizes emotional distance and lack of involvement.
The best synonym to use will depend on the specific situation and what you want to emphasize.