I need to declined a job offer soon, probably today. My question is how much information should I give them when declining the offer. Should I share the details that went into my decision making or should it lack details at all?
It's hard to answer this without knowing what the details are that you're considering sharing, so I'll give you a way to decide whether any particular detail needs to be shared:
What will the company do as a result of the information you give them?
For example, if your reason is "you didn't offer enough money" then they might come back with more money. However in that case an out of band communication saying "I have to decline that offer but would accept one for X more a year" is smarter than declining with reasons.
If your reason is "I love everything about the company but cannot move from A and just can't handle the commute to B" then should they open an office in B, or the commute improve in some way, you guys might want to talk again and having it on file that only geography kept you from accepting might be good.
If your reason is "the third person I met was insanely rude and would have resulted in an immediate trip to HR to file harassment documents if I worked there, and working with this abusive individual is something I can't even consider" then you might consider helping whoever has to work with that person by reporting this, and how it cost the company someone they wanted to hire (you.) Further, if that person gets fired perhaps you would work there after all. Due to the sensitive aspect I would probably do this in a phone call not a letter.
And so on. For any particular piece of information, consider its use. They don't care how hard a decision it was for you, where your family lives, or whether you like to ride your bike. They might be prepared to adjust some parts of the offer, but that's best done before you decline. You might like to tell them that their shabby office furniture (or overly gleaming and expensive office furniture) sealed your final decision to take a different offer, but they really will gain nothing from learning that from you, so don't bother.
Should I share the details that went into my decision making or should it lack details at all?
Nope. You should not. You should let vagueness speak volumes. Such as saying, “While I appreciate the offer, I believe my skills are not exactly in line with your organization’s overall goals.” Or something along those lines.
But in general, a potential employer is really owed nothing when it comes to an explanation as to why you are declining an offer. This is like any other relationship, but is ultimately a business transaction. So if you do not want to work for them, they might take a “we don’t take no for an answer” stance, but it’s best to just walk away from folks like that. If they can’t respect your needs at this stage, how will they respect your needs later on?
The only real exception to this rule is if you were somehow interested in working for the company in the future, but not in the position offered. If so I would say something like, “While I appreciate the offer—and respect your organization—I am not too sure my skills would be a good fit for this position at this time.”