Why is the 'L' in detailed not doubled?
Solution 1:
In British English, in which L is often doubled before inflectional suffixes, it is normally only doubled when there is just one of the letters A, E, I, O, U directly before the last consonant before the suffix:
- detail[ed] (two of these letters, A & I, behind the suffix - no double L).
- travel[led] (just one E before the suffix, therefore double L).
L is an exception in British English. It doesn't behave for doubling purposes like other consonants, for example P, T, K, B, D, G. Importantly, it does not matter whether the syllable it occurs in is stressed or not.
American English does not have this L doubling rule. In American English, as with other consonants, the L is only doubled if the syllable before the suffix is stressed.
There are also, as always with English spelling, exceptions to the rule.
Solution 2:
Here are some good rules of thumb for knowing when to use a doubled consonant (from David Crystal's book Spell it Out). The OP's example either has a long vowel or falls under exception 1(c). Interestingly the French cognate of the word (détaillé) has the double l.
- (a) To indicate that the previous vowel is short (hopping vs. hoping), except (b) in words consisting of a single closed syllable ending in nasals or plosive (pan, not pann, but scuff and bill), and except (c) when the short vowel is written with two letters (breaded), and except (d) when the consonant itself is written with two letters (dishes), and except (e) when the consonant is v or x (axes, devil).
- Except not in sets of related words formed from adding (or not) Latinate suffixes (athlete vs. athletic).
- Except not in some later French and Latin loan words (proper vs. pepper)