Swift: NumberFormatter with more decimal places

I am using the NumberFormatter class to format crypto currencies, such as Ethereum, which can support up to 18 decimal places. NumberFormatter seems to start rounding after 15 decimal places, however, even if it's given a NSDecimalNumber. I presume this happens because it's storing the value as a Double under the hood. Is there a way to get around this so that I can format Decimal's with more precision?