Year	Percent
1987	85.77897
1988	85.41105
1989	85.43194
1990	85.08233
1991	83.82891
1992	83.4999
1993	82.84702
1994	83.35282
1995	83.1009
1996	82.55637
1997	81.75407
1998	81.56329
1999	81.47976
2000	82.328
2001	82.66609
2002	82.98226
2003	82.46525
2004	82.24882
2005	82.13498
2006	82.55368
2007	82.11307
2008	81.94446
2009	81.2922
2010	81.44614
2011	82.09045
2012	82.1906
2013	82.37276
2014	82.43292
2015	82.51975
2016	82.84733
2017	82.6157