Why does an explicit cast to ‘decimal’ call an explicit operator to ‘long’?

Consider the following code:

class Program
{
    public static explicit operator long(Program x) { return 47; }

    static int Main(string[] args)
    {
        var x = new Program();
        Console.WriteLine((decimal) x);
    }
}

To my surprise, this outputs 47; in other words, the explicit operator long is called even though the cast is to decimal.

Is there something in the C# spec that explicitly says that this should happen (if so, where exactly) or is this the result of some other rule(s) I’m missing?