"Pons" is one of the bigger publishing houses for dictionaries in Germany, and their free website
http://de.pons.com/ is quite good. It also includes verb patterns. It might include a few idioms, but in general I don't know of any dictionary that actually translates idioms. E.g., when I need to look up modern English idioms, I usually use an English-English site like
http://www.urbandictionary.com/
Googling for "deutsche idiome" finds a number of German-German sites, e.g.
http://www.redensarten-index.de, but I haven't used them, so I can't say how good they are.
For natural translation of expressions,
http://www.linguee.de/ (correlated public texts in several languages) is very helpful.