The Idaho Secretary of State has released registration data as of July 6, 2017. The percentages: Republican 49.78%; Democratic 11.00%; Libertarian .79%; Constitution .37%; independent and other 38.06%.
In November 2016, the percentages were: Republican 48.27%; Democratic 10.64%; Libertarian .64%; Constitution .32%; independent and other 40.13%.
Idaho registration statistics appear to lag events, and may vary by county. Idaho also has election day registration, The monthly increases reported in October, November, December, and January were: +12K, +28K, +41K, +15K. Overall, total registration increased by 12.4% during that four month period. If one wanted registration as of the “2016 General Election”, the January 2017 report would be the most useful.
The purge occurred over two months. This showed up in March 2017, and was -8.3%. Purges would be of voters who could not be contacted (before the 2014 general election) and then did not vote in 2014 and 2016. Some of these may have left the state, or perhaps moved within the state and not re-registered.
Idaho is still showing the effect of the switch to partisan registration. Idaho lets each party determine who may vote in their primaries, and like in Alaska they may select which parties may vote in their primary. The Democrats let anyone, including Republicans and unaffiliated, vote in their primary. The Republicans permit only Republicans, but state law permits unaffiliated voters affiliate on election day.
At the time of the March 2016 presidential primary, Republican registration increased 60K, unaffiliated decreased by 40K, and new registrants were about 20K. That is most of a 20% increase in Republican registrations (in one month) was due to previously unaffiliated voters affiliating.
Libertarian registration declined by about 1/9, apparently due to voters wanting to vote in the Republican primary. There was also a small switch of unaffiliated to Republican at the May primary, but Idaho had few contested races above the legislative level.
At the purge, 13.0% of unaffiliated voters were removed from the rolls, compared to 4.6% of Republicans, 7.3% of Democrats, 8.1% of Constitution’ers, and 8.5% of Libertarians.
Unaffiliated registrations tend to be more stale, as they had not voted in a primary. Republicans have quite likely voted in a primary in the past few years.
Since March, changes have been minimal