how to change the line-thickness of a textspanner

[Stefan Thomas]

Dear community,
I tried to change the line-thickness of a textspanner, but without success.
Does someone have an idea?
Here is my snippet:
\version "2.12.2"
music = { c'1 \startTextSpan c' \stopTextSpan }

\new Staff { 
   \override TextSpanner #'style = #'line
  \override TextSpanner #'line-thickness = #0.5 
  \override TextSpanner #'bound-details #'left #'text = "Hello"
  \music 
 \override TextSpanner #'line-thickness = #15 %it seems to do nothing!
\music

}