sum of columns no fetching from database

Here i want to get the sum of balance of selected customer_id.for example here the customer_id of 19 is having an sum of 120 but the result is not fetching. here is my controler

public function make_payment()
{
   $data['active_mn']='';
   $data['active_mn']='';
   $data['result']=$this->Account_model->view_order_id($code);
   $customer_id=$this->input->post('customer_id');
   $data['details'] = $this->Account_model->order_details($customer_id)->row(); 
   $this->load->view('make_payment',$data);

  }

This is my model

public function order_details($customer_id)
{
    $this->db->join('payment','payment.order_id=services.id');

    $this->db->select('services.*,payment.balance,payment.actual_amount,payment.customer_id');
    //$this->db->select_sum('payment.balance');
    $this->db->order_by('payment.id','desc');
    $query = $this->db->get_where('services',array('customer_id' => $customer_id));
    retu $query;
}

this is my payment table..

    id  order_id customer_id    actual_amount   paid_amount     balance   type
    5      9         19           650.00          600.00        50.00     Cash
    6      8         35           600.00          500.00        0.00      Cash
    7      7         15           750.00          750.00        0.00      Cash
    8      5         16           800.00          400.00        400.00    Cash
    9      4         35           400.00          300.00        100.00    Cash
    10     3         19           500.00          450.00        50.00     Cash
    11     2         15           100.00          100.00        0.00      Cash
    12     1         16           1200.00         1000.00       200.00    Cash
    18     10        19           100.00          80.00         20.00     Cash

this is my services table

id  code    customer_id     estimated_amount    
1   ORD00001    16               100    
2   ORD00002    15               324    
3   ORD00003    19               11     
4   ORD00004    35               54     
5   ORD00005    16               400    
7   ORD00007    15               555    
8   ORD00008    35               12