It's not an idiom, it's correct. 'Turning' is perfectly OK as a gerund. 'ing' appears in three places in Engish - as a gerund, as a progressive tense - both of which involve adding 'ing' to a verb, and as a noun-forming modifier for which I cannot find the correct terminology. The latter case involves adding 'ling' to a non-verb to produce a commonly-diminutive noun. Examples include underling, sapling, darling, sidling (which produced the verb sidle as a back formation). It used to be side-ling - i.e. someone who stands at your side as opposed to a subordinate under-ling. But of course given a variety of words ending in 'ing' it isn't always apparent that some were fromed from the 'ling'-route rather than the verbal 'ing'-route. The 'l' is a useful indicator, but not 100% reliable.