The idea/theme of "The Summer of a Beautiful White Horse" entails honesty, honour, and trust.

Honesty:
- The story revolves around a young Armenian girl named Voski, who is tasked with selling her family's white horse to pay off their debts.
- Voski's honesty is tested when a man steals the horse from her and she must decide whether to tell the truth to her family or keep it a secret.
- The theme of honesty is also seen in the character of Uncle Khosrove, who refuses to sell the horse to anyone who he feels is not deserving of it.

Honour:
- The white horse is seen as a symbol of honour and pride for Voski's family, and they are determined to sell it to the right person who will treat it with respect.
- Uncle Khosrove is a man of honour who values tradition and the importance of keeping one's word.
- The story highlights the importance of honouring one's commitments and staying true to one's values.

Trust:
- Voski's family must trust her to sell the horse and bring back the money to pay off their debts.
- Voski must also trust the people she encounters, including a group of strangers who help her when the horse is stolen.
- The importance of trust is also seen in the relationship between Uncle Khosrove and his nephew Aram, who must trust each other to make the right decisions.

Overall, the idea/theme of "The Summer of a Beautiful White Horse" encompasses honesty, honour, and trust, highlighting the importance of these virtues in one's life.