2017 2016 2015 2014 2013 2012 2011 2010
January $2.647 $2.473 $3.457 $3.980 $4.145 $3.977 $3.375 $2.865
February $2.789 $2.328 $3.188 $4.074 $4.099 $4.006 $3.541 $2.991
March $2.768 $2.203 $3.082 $4.340 $4.224 $4.131 $3.748 $2.930
April   $2.252 $3.269 $4.308 $4.164 $4.260 $4.046 $3.052
May   $2.316 $3.098 $4.194 $4.019 $4.261 $4.191 $3.174
June   $2.435 $3.151 $4.149 $3.919 $4.108 $4.189 $3.200
July   $2.534 $3.116 $4.078 $3.910 $3.891 $4.063 $3.084
August   $2.510 $3.004 $4.032 $3.938 $3.851 $4.042 $3.027
September   $2.442 $2.805 $3.965 $3.976 $4.034 $4.000 $3.044
October   $2.492 $2.680 $3.900 $4.033 $4.174 $3.966 $3.025
November   $2.554 $2.639 $3.761 $3.961 $4.138 $3.921 $3.160
December   $2.562 $2.597 $3.625 $3.913 $4.174 $4.060 $3.258