No quote ''
<a href ={% url user_details x.id %} class='btn btn-primary' style="float: right;" >
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</a>
Another your url
url(r'^User/(?P<userid>\d+)/$', 'search.views.user_detail', name='user_details'),
Be carefull, after Django 1.5, use must use quotes. I came across this solution and tried it, got an error. I'm using Django 1.6 and you need the quotes:
<a href ={% url 'user_details' x.id %} class='btn btn-primary' style="float: right;" >
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</a>
hope it helps.