public function getPrefixForNamespace( $namespace )
	{
		if (
			  ! isset( $this->cacheNamespaces ) ||
			  count( $this->cacheNamespaces ) != count( $this->instance_namespaces )
		   )
		{
			$this->cacheNamespaces = array_flip( $this->instance_namespaces );
			// If the default namespace is one of the standards ones then the prefix may be missing
			$standardNamespaces = array_flip( XBRL_Constants::$standardPrefixes );
			if ( isset( $this->instance_namespaces[''] ) )
			{
				if ( isset( $standardNamespaces[ $this->instance_namespaces[''] ] ) )
				{
					$this->cacheNamespaces[ $this->instance_namespaces[''] ] = $standardNamespaces[ $this->instance_namespaces[''] ];
				}
			}
		}

		return isset( $this->cacheNamespaces[ $namespace ] )
			? $this->cacheNamespaces[ $namespace ]
			: false;
	}