The D2000's have a slight amount of leakage. I tested this out by placing the headphones over a full tissue box (I'm assuming the tissues inside damp the noise similar to the way your head does). The leakage is far less than an open headphone, but is somewhat more than a fully closed headphone. Basically, in a quiet environment someone next to you might hear some light, washy noise coming out of the headphones, but they can't hear the music perfectly like they would be able to with an open headphone.
As some people have mentioned elsewhere, the D2000s are very slightly open. There is a small gap between the silver frame and black portion of the cups. If you place your ear against the gap, you can hear your music perfectly. But again, you literally have to have your ear pressed against it to hear it.
On low volume they don't leak at all.
On medium - leak is significant.
On high volume - epic amount of leakage, i mean EPIC - whole building will be able to hear your music.
Denon cans meant to be listened LOUD. On medium volume they're not that impressive, - your head will not vibrate with subbass they capable to produce.
So go for something else, like DT770.