Fair Value Measurement - Additional Information (Details) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair value, assets transfer between the three fair value hierarchy levels	$ 0	$ 0
Fair value, liabilities transfer between the three fair value hierarchy levels	$ 0	$ 0

Fair value liabilities transfers between the three levels amount.

No definition available.

Name:	bsm_FairValueLiabilitiesTransfersBetweenTheThreeLevelsAmount
Namespace Prefix:	bsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration