It depends on when you began receiving the U.S. Social Security which exemption you qualify for. If you started receiving SS prior to 1996, you should read the following rules which apply to the 50% exemption.
From CRA:
Under the Canada-United States tax treaty, you can claim a deduction equal to 15% of the U.S. Social Security benefits, including U.S. Medicare premiums which you reported as income on line 115.
If you have been a resident of Canada and have received U.S. Social Security benefits continuously during the period starting before January 1, 1996, and ending in 2017, you can claim a deduction equal to 50% of the U.S. Social Security benefits received in 2017.
This 50% deduction also applies to you if you are receiving benefits related to a deceased person and you meet all the following conditions: