In the beginning of the Amidah, we identify "avoteinu" our forefathers and we list them as Avraham, Yitzchak and Yaakov. But in the Haggadah, we begin by saying "Mitchilah, ovdei avodah zara hayu avoteinu" and this is expanded on by citing a pasuk from Yehoshua 24:2-4 which reads (from the chabad site)
And Joshua said to the whole nation, "Thus said the Lord God of Israel, 'Your fathers dwelt on the other side of the river from earliest time, Terah, the father of Abraham, and the father of Nahor; and they served other gods.
The fathers here (avoteichem) are Terach, Avraham and Nachor. Haran is not listed (dealt with in part here).
While I know that Terach is Avraham's father so, strictly speaking, he is an ancestor, I never thought of him as one of the avot. Nachor and Avraham took Haran's daughters as wives, so Haran is also an ancestor as is Nachor.
If the verse is talking about the well known three avot (Avraham, Yitzchak, Yaakov) then they weren't idol worshippers. If it is talking about Terach and Nachor, while they worshipped idols, they weren't "avot" in the sense I understand. If Haran isn't included because he might not have been clearly an idol worshipper, why is Avraham listed?