public function getPrefixForDocumentNamespace( $namespace )
	{
		if ( is_null( $this->cacheDocumentNamespaces ) )
		{
			$this->cacheDocumentNamespaces = array_flip( $this->instance_xml->getDocNamespaces() );
			// If the default namespace is one of the standards ones then the prefix may be missing
			$standardNamespaces = array_flip( XBRL_Constants::$standardPrefixes );
			$docNamespace = array_search( '', $this->cacheDocumentNamespaces );
			if ( $docNamespace )
			{
				if ( isset( $standardNamespaces[ $docNamespace ] ) )
				{
					$this->cacheDocumentNamespaces[ $docNamespace ] = $standardNamespaces[ $docNamespace ];
				}
				else
				{
					// Get the namespace for the respective namespace
					$taxonomy = $this->getInstanceTaxonomy()->getTaxonomyForNamespace( $docNamespace );
					if ( $taxonomy )
					{
						$this->cacheDocumentNamespaces[ $docNamespace ] = $taxonomy->getPrefix();
					}
				}
			}
		}

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