You can cash out wherever you bought the BTC. It's simply the reverse process. If you decide to use Coinbase, consider doing it on Gdax instead. You should get better rates there and since it's part of the same company, your Coinbase account can be used on Gdax as well.
Be prepared to face some extra KYC/AML questions as you try to withdraw your funds to your bank account. $1M+ will raise more flags than what you originally spent on buying the coins. In addition, notify your bank in advance of the incoming transfer to prevent them from freaking out.
