Summary of Significant Accounting Policies - Income Taxes (Details)	12 Months Ended
Dec. 31, 2019
Income Taxes
Valuation allowance on deferred tax assets (as a percent)	100.00%

The percentage of valuation allowance on the deferred tax assets.

No definition available.

Name:	mdgl_DeferredTaxAssetsPercentageOfValuationAllowance
Namespace Prefix:	mdgl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration