public function createCredit()
    {
        $copy = $this->copy();
        $detailsCopy = new $this->details;
        foreach ($copy->details as $detail) {
            $detail->setData(array(
                'amount' => $detail->amount * -1
            ));
            $detailsCopy->append($detail);
        }
        $copy->setData(array(
            'details' => $detailsCopy
        ));
        return $copy;
    }