From an op ed piece in the Washington Post by Senators Bill Cassidy (R-LA) and Tim Kaine (D-VV):
… We propose creating an additional investment fund — in parallel to the trust fund, not replacing it — that would be invested in stocks, bonds and other investments that generate a higher rate of return, helping keep the program from running dry.
We estimate that it would take a $1.5 trillion up-front investment into the fund to get it going, and we propose giving the fund 75 years to grow. The Treasury would temporarily shoulder the burden of providing benefits to Social Security beneficiaries — but when the new fund’s 75 years are up, it would pay the Treasury back and supplement payroll taxes to help fill the future gap.
The result? The consistent delivery of Social Security benefits for generations of Americans, and a reduction to the United States’ long-term indebtedness by up to 20 percent. …