Insert blank line in a file

Shell Programming and Scripting
1

I have a file with data as below :

Heading 1
-------------
Heading 1 data1
Heading 1 data2
Heading 1 data3
Heading 1 data4
Heading 2
-------------
Heading 2 data1
Heading 2 data2
Heading 2 data3
Heading 2 data4
Heading 3
-------------
Heading 3 data1
Heading 3 data2
Heading 3 data3
Heading 3 data4

I want to insert a blank line after each set of Heading.
ie; The data should look as below.

Heading 1
-------------
Heading 1 data1
Heading 1 data2
Heading 1 data3
Heading 1 data4

Heading 2
-------------
Heading 2 data1
Heading 2 data2
Heading 2 data3
Heading 2 data4

Heading 3
-------------
Heading 3 data1
Heading 3 data2
Heading 3 data3
Heading 3 data4

How can we do this using awk/sed.
We can use --- for pattern search if required.

Thanks

2 
awk 'NR>1 && NF==2 {print RS $0; next}1' infile
3

Thanks alot ! This works fine !