documentclass{article}

usepackage[american]{babel}
usepackage{tabularx}
usepackage{booktabs}

begin{document}
begin{tabular}{rr}
  toprule
  multicolumn{1}{c}{$alpha$} & multicolumn{1}{c}{$alpha=0.99999$} \
  midrule
  10     & 69.681   \
         & 901.741  \
  1000   & 893.630  \
         & 82.806   \
  bottomrule
end{tabular}
end{document}

documentclass{article}
usepackage{tabularx}% http://ctan.org/pkg/tabularx
usepackage{booktabs}% http://ctan.org/pkg/booktabs
begin{document}
begin{tabular}{rc}
  toprule
  multicolumn{1}{c}{$alpha$} & $alpha=0.99999$       midrule
  10     & phantom{0}69.681                & 901.741        1000   & 893.630               & phantom{0}82.806         bottomrule
end{tabular}
end{document}

documentclass{article}

usepackage[american]{babel}
usepackage{tabularx}
usepackage{booktabs}

begin{document}
begin{tabular}{rr}
  toprule
  multicolumn{1}{c}{$alpha$} & multicolumn{1}{c}{$alpha=0.99999$}       midrule
  10     & 69.681                & 901.741        1000   & 893.630               & 82.80600000000000000000         bottomrule
end{tabular}
end{document}

documentclass{article}
usepackage{booktabs}
usepackage{dcolumn}

newcolumntype{M}{D{.}{.}{3.3}}

begin{document}

begin{tabular}{rM}
toprule
multicolumn{1}{c}{$alpha$} & multicolumn{1}{c}{$alpha=0.99999$} \
midrule
10     & 69.681   \
       & 901.741  \
1000   & 893.630  \
       & 82.806   \
bottomrule
end{tabular}

end{document}

documentclass{article}
usepackage{booktabs}
usepackage{siunitx}

begin{document}

begin{tabular}{
  r
  S[
      table-format = 3.3,
      input-symbols=.,
  ]
  }
    toprule
    multicolumn{1}{c}{$alpha$} & multicolumn{1}{c}{$alpha=0.99999$} \
    midrule
    10     & 69.681   \
           & 901.741  \
    1000   & 893.630  \
           & 82.806   \
    bottomrule
end{tabular}

end{document}