Make all <li> same width as the widest

I've got this menu wich is setup inline and has dropdowns.
The inner ul has a background.
Each dropdown li has a :hover that changes the background of the li:

<div id="navMain">
    <ul>
        <li><a href="#nogo">Forside</a>
            <ul>
                <li><a href="#nogo">1111111111111</a></li>
                <li><a href="#nogo">Link 1-2</a></li>
                <!-- etc. -->
            </ul>
        </li>
        <li><a href="#nogo">Om Os</a>
            <ul>
                <li><a href="#nogo">Link 2-1</a></li>
                <li><a href="#nogo">Link 2-2</a></li>
                <!-- etc. -->
            </ul>
        </li>
        <li><a href="#nogo">Link 3</a>
            <ul>
                <li><a href="#nogo">Link 3-1</a></li>
                <li><a href="#nogo">Link 3-2</a></li>
                <!-- etc. -->
            </ul>
        </li>
    </ul>
</div>

Problem is, that when one of the submenu li is longer than the others, it will only expand itself, and not the other li ofcourse.
This results in the :hover effect having different lengths.

So how would i make all li in each inner ul the same size as the widest one?

Here you can find the CSS if needed.

Solution

Here. Notice I added a class to your menu li's and that I added a body background to your css, because I couldn't notice your menus. Finally the trick is done by making the li elements 100% width

body {
  background-color: green;
}

.menu li {
  width: 100%
}

#navMain {}

#navMain ul {
  padding: 0;
  margin: 0;
  z-index: 2;
}

#navMain ul li {
  margin-right: 5px;
  text-align: center;
}

#navMain li a {
  display: block;
  text-decoration: none;
  color: white;
  padding-left: 10px;
  padding-right: 10px;
}

#navMain li a:hover {
  background-color: black;
}

#navMain ul li:last-child {
  margin-right: 0px;
}

#navMain li {
  position: relative;
  float: left;
  list-style: none;
  margin: 0;
  padding: 0;
  font-size: 17px;
  font-weight: bold;
  color: #fff;
}

#navMain ul ul {
  position: absolute;
  top: 20px;
  visibility: hidden;
  background-image: url(img/alphaBg.png);
}

#navMain ul li ul li {
  font-size: 12px;
  margin-right: 0px;
  text-align: left;
}

#navMain ul li ul li:first-child {
  padding-top: 5px;
}

#navMain ul li:hover ul {
  visibility: visible;
}

<html>

<head>
</head>

<body>
  <div id="navMain">
    <ul>
      <li><a href="#nogo">Forside</a>
        <ul class="menu">
          <li><a href="#nogo">1111111111111</a></li>
          <li><a href="#nogo">Link 1-2</a></li>
          <li><a href="#nogo">Link 1-3</a></li>
          <li><a href="#nogo">Link 1-3</a></li>
          <li><a href="#nogo">Link 1-3</a></li>
          <li><a href="#nogo">Link 1-3</a></li>
        </ul>
      </li>
      <li><a href="#nogo">Om Os</a>
        <ul class="menu">
          <li><a href="#nogo">Link 2-1</a></li>
          <li><a href="#nogo">Link 2-2</a></li>
          <li><a href="#nogo">Link 2-3</a></li>
        </ul>
      </li>
      <li><a href="#nogo">Link 3</a>
        <ul class="menu">
          <li><a href="#nogo">Link 3-1</a></li>
          <li><a href="#nogo">Link 3-2</a></li>
          <li><a href="#nogo">Link 3-3</a></li>
        </ul>
      </li>
    </ul>
  </div>
</body>

</html>