Derivative Financial Instruments Credit Facility and Derivative Counterparties (Details)	Sep. 30, 2018
Derivative Instruments Not Designated as Hedging Instruments [Abstract]
Percentage of Proved Property Secured for Credit Facility Borrowing	85.00%

Percentage of Proved Property Secured for Credit Facility Borrowing

No definition available.

Name:	sm_PercentageofProvedPropertySecuredforCreditFacilityBorrowing
Namespace Prefix:	sm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant