Some people set the goal of retiring with $1 million. Others might aim for $2 million or more.

The reality is that the amount of money you end up needing in retirement will hinge on factors that include your health and your desired lifestyle. If you're plagued by medical issues, you'll need a higher income to cover your various healthcare costs. And if you hope to travel the globe and indulge in your share of fine dining, you'll need more money than someone who plans to spend their days gardening and exploring good books.

But either way, it pays to build as robust a nest egg as possible for your senior years to give yourself options and avoid financial stress. And to amass a decent amount of wealth, you can't leave your retirement savings to sit in cash. Rather, you'll need to invest that money savvily. And there's one specific investment that could be a solid bet.

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Invest in the broad market

There are different strategies you can adopt when it comes to investing for retirement. But if you want to take the easy way out -- which, to be clear, wouldn't be a bad thing at all -- then you may want to put your long-term savings into broad market index funds.

Index funds are passively managed and have the goal of matching the performance of whatever benchmark they're tied to. A good bet for your retirement may be to load up on S&P 500 index funds. That way, you'll effectively be putting your money into the stock market on a whole, which means you'll benefit when it's up and see your portfolio value decline when it's down.

But all told, throughout its history, the stock market has had more up years than down years. (If that weren't the case, fewer people would invest in it.) And so putting your money into the broad market is a great way to maintain a diverse investment mix without having to do too much research or thinking.

Another benefit to buying broad market index funds? You won't lose a lot of money to fees, which could be the case if you were to load up on actively managed mutual funds.

Set yourself up for success

Whether you have modest plans for retirement or lofty ones, the right investment strategy could help you achieve your goals. It pays to think about investing your long-term savings in broad market index funds.

And if you're not convinced, it may help you to know that Warren Buffett, one of the most successful investors of our time, thinks broad market index funds are a great bet for the average saver. Though index funds won't let you beat the broad market, they do let you benefit from its performance. And that alone could set the stage for a millionaire retirement, especially if you begin funding an IRA or 401(k) plan from an early age and continue doing so consistently until your time in the workforce comes to an end.