total market capitalisation of the New York Stock Exchange

[rcyeh]

One question is:
What was the total market capitalisation of the New York Stock Exchange in January of 2015, in billions of dollars?

The exchange itself is merely a marketplace, owned by one of the companies listed on the exchange. For clarity, this question should be re-phrased to:
What was the total market capitalisation of all companies listed on the New York Stock Exchange in January of 2015, in billions of dollars?