In XSLT 1.0 the conversion will be done automatically, or you can call the number() function.
In XSLT 2.0 you can call the number function. This will give you a double. If you want an integer, you can call the xs:integer() function.
Personally, in 2.0 I would do the string manipulation using a regular expression. I think your expression can be replaced by tokenize(securityKey, ':').
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference