I have my config:
Your property file location is classpath:idm.properties
This is rather unusual, it means that idm.properties must be located either at the top level of WEB-INF/classes or at the top-level of one of the jars inside WEB-INF/lib. Usually it's good practice to either use a dedicated folder for properties or keep them close to the context files that use them.
So my suggestion is this: Is your properties file perhaps next to your context file? If so, it's not on the classpath (see this question: Is WEB-INF in the CLASSPATH?).
The classpath: prefix maps to a ClassPathResource, but you probably need a ServletContextResource, and you'll get that from a WebApplicationContext using the syntax without prefix:
Reference:
ApplicationContext types handle resources without prefix) mechanism)