I grew up in Boise and have spent my adult life living in either Salt Lake City or Boise. Boise does have a lot of mormons but it isn't anywhere close to being like Utah.
Rodolfo is right-- stay clear of living in Meridian or Eagle and you should be fine. Even then, there are plenty of non-mormons even in those areas and in no place around Boise are mormons "the majority." You'd have to go to Eastern Idaho to run into that.
I loved living in Boise. It's a great small city and housing prices right now are just incredible. Good luck!
I'll second the comments about Eagle and Meridian. I would guess that Boise (proper) is only about 15% Mormon and Meridian is more like 30%.
The stakes in the Boise city limits are all shrinking. The wards that cover the north end and near downtown are all but dead. Part of that is just demographics as the young families move out to Meridian and Eagle and points west.
Of course, that demographic is also reflected in the schools. Boise schools are on a gradual decline, while Meridian can't build schools fast enough.
email me at mac83704 at yahoo dot com if you have specific questions. I currently live in Boise (15 years) and I'd be happy to help.
Meridian Idaho and even in Eagle (Boise area) gives the illusion that there are tons of them and they dominate the area only because there are so many temples within a one mile area but mostly the temples were built before the economy crashed and now they sit amongst empty subdivisions. I have been told by elementary school age parents if you have younger kids in elementary school I have heard the LDS kids won't play with your non-mormon child. Perhaps they don't want their children tainted by us lowly gentiles? Strange that most of the high schools have a seminary building next to them and many of the LDS kids only hang out with their own. Though most are nice there are some that are "holier than though" and make it miserable for some of the non-mormons by making them feel as though they are "not good enough". But I chalk that up to some kids are just mean and naive and others aren't. Don't open up a business here in Boise because the LDS community won't go to you. They do business in the church like others do business on the golf course. (ironic since Jesus never liked business being done in the temple). Work for HP or Micron or some other large company and you'll be ok.