Year	% of total
1987	100
1997	98.3
1998	98.8
1999	98.7
2000	98.8
2001	98.9
2002	98.8
2003	98.9
2004	98.6
2005	98.8
2006	98.7
2007	97.8
2009	98.7
2010	98.5
2011	98.5
2012	98.2
2013	96.3
2014	95.9
2015	95.4
2016	94.4
2017	95.1
2018	95.3