Insurance for Buy and Hold?

[Alex SImon]

I'm going through the purchase process for my first two buy and hold rentals in Philly right now, but I have no idea what kind of insurance policies I need to carry.  I've been going through the forums but I can't seem to find any breakdown of what exactly I need to be looking for.  A lot of people mention an Umbrella policy, but I have no idea whether that's all I need or if I need that plus something else.

Can anyone help, either by explaining the subject or pointing me to a resource where I can read up on it myself?