Recently I came across a text referring to an article on the newspaper "Der Spiegel" as "Ein Artikel aus dem Spiegel".
This had me confused. Being under the impression that "Der Spiegel" is simply the name of the paper and hence a proper noun, I expected it to not be declined under any circumstances.
Is this normal for German? Are proper nouns always declined as if the words composing them were taken out of context? Are there rules as to when and how to decline proper nouns?