law unto oneself
A person who is totally independent, especially one who ignores established rules. For example, You can't tell Marge how to punctuate; she's a law unto herself. [Second half of 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.