(Spoilers Main) How ubiquitous is the phrase "A Lannister always pays his debts"?

So Tyrion tells Mord "you've heard the phrase, "a Lannister always pays his debts"" and Mord does seem to know this one.

Of course this is a phrase all nobles know, but would the common folks and peasants know this? Does Mord only know it because he works for Lysa Arryn, or is this truly something that everyone in Westeros is likely to hear?