How can i get the Idoc Number for the Billing Document in SAP CRM system

HI,

I have Requiremnnt where the after the creation of Invoice , a Idoc is genarated for those respective Billing document and as of now i just want to know how can i get the Idoc Number for tht respective billing docs as im new to SAP CRM im uanble to find the Idoc Numbers.Any help will be highly appreciated .