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.