Label Arrow with tikz

Question

This is a super simple problem, but I can't figure out how to add a label to an arrow between two nodes. Right now, the code I have seems to add an extra node in between the nodes I wish to connect and labels that node what I want to label the arrow (and it makes a mess). How can I do this properly?

\begin{center}
\begin{tikzpicture}[scale=2, every node/.style={circle, fill=blue!20, minimum size=15mm}]
\node (A) at (0, 1) {A};
\node (B) at (-1, -.5) {B};
\node (C) at (1, -.5) {C};
\draw [thick, ->] (A) to node[left] {-10} (B);
\draw [thick, ->] (A) to node[left] {15} (C);
\draw [thick, ->] (B) to node[left] {20] (C);
\end{tikzpicture}
\end{center}

Answer 1

Too long for a comment. With every node/.style={...} you instructed TikZ to draw all nodes with a fat bullet, but it seems like you don't want this to happen for the numbers. So one way to go is

\documentclass[border=2mm,tikz]{standalone}
\begin{document}
\begin{tikzpicture}[scale=2, fatnode/.style={circle, fill=blue!20, minimum size=15mm}]
\node[fatnode] (A) at (0, 1) {A};
\node[fatnode] (B) at (-1, -.5) {B};
\node[fatnode] (C) at (1, -.5) {C};
\draw [thick, ->] (A) -- (B) node[midway,above left] {-10};
\draw [thick, ->] (A) -- (C) node[midway,above right] {15};
\draw [thick, ->] (B) -- (C) node[midway,above] {20};
\end{tikzpicture}
\end{document}