from the US Canada tax treaty: 

"Pensions, Annuities, Social Security, and Alimony

Under Article XVIII, pensions and annuities from Canadian sources paid to U.S. residents are subject to tax by Canada, but the tax is limited to 15% of the gross amount (if a periodic pension payment) or of the taxable amount (if an annuity). Canadian pensions and annuities paid to U.S. residents may be taxed by the United States, but the amount of any pension included in income for U.S. tax purposes may not be more than the amount that would be included in income in Canada if the recipient were a Canadian resident."          here is the link: https://www.irs.gov/publications/p597

I would put the taxable amount under other income