Write SQL query to solve given problem: State the documentary film titles with longest length.. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T1.title FROM film AS T1 INNER JOIN film_category AS T2 ON T1.film_id = T2.film_id INNER JOIN category AS T3 ON T3.category_id = T2.category_id WHERE T3.name = 'documentary' ORDER BY T1.length DESC LIMIT 1