The data stored in Google Docs is pretty darn secure because as a first measure a Google Account is required to login and access any of your Google Docs information and Data.
Besides that Google has a lot of other security measures in place to protect your Google Docs data as well as other data and information that is hosted on your Google Accounts and in their data centers.
Many people use Google Docs and it's been pretty secure with no problems so as long as you constantly change your Google Account password which you should do anyway on Google and other accounts then your Google Docs data should be safe and secure.