I believe it is personal preference influenced by locality, culture, and conversational context. For example, the word envelope can have two different pronunciations in American English: one where the en- portion can have a French-like pronunciation:
and another that uses the strict English pronunciation of en-, which sounds like the letter n. My mother prefers the French-like pronunciation; I prefer the strict English pronunciation. We're from the same family of people who grew up in the same area of the country, and we pronounce it differently.
In the UK, I have heard people add letters to words where no such letters exist. For example, to pronounce the word:
some British people will make the u a strict letter pronunciation and add an i where none exists:
al-yoo-MIN-ee-um (as if the word were spelled alyuminium)
Most Americans pronounce aluminum with the stress on the second syllable and do not add a letter:
Another word that gets interesting treatment by the British is:
A British person can pronounce lieutenant where the first t sounds more like an f:
An American will use a hard t and stress the second syllable:
Canandians and Americans pronounce the letter combination ou in some words differently. An American can pronounce ou as if the u sounded like a w:
A Canadian can make the ou sound like the ou in soup:
about: a-BOUWT or a-BOOT
Oddly enough, Americans and Canadians pronounce soup the same way.
So pronouncing detail as DE-tail or de-TAIL is matter of preference. As long as people understand that detail in one context is similar to detail in another context, it doesn't matter a great deal how it is pronounced.