Eloquent: Relationships with 3 tables in laravel 5

I have three models namely Header, Details, Item. The Header have id, customer_id, the Details have id, header_id (FOREIGN KEY), and item_id (FOREIGN KEY), and the Item have id, name. Now, I want to relate that tables using laravel eloquent relationships. I've been able to do that with:

class Details extends Model
{
    public function item() {
        return $this->belongsTo('App\Item', 'bill_item_id');
    }

    public function header() {
        return $this->belongsTo('App\Header', 'header_id');
    }
}

The problem is in my controller, I want to get the details but details don't have customer_id.

$detail = Details::where('customer_id', $id)->get();
$detail->load('header', 'item');

The customer_id field is in the header model. if I get all the details, it's working fine but I want to get specific customer.

via Chebli Mohamed