Commitments and Contingencies (Details) $ in Millions	Dec. 31, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Commitments to invest with an Affiliate (up to)	$ 41

Amount of commitments to fund investment activity.

No definition available.

Name:	bsig_CommitmentsToFundInvestmentActivity
Namespace Prefix:	bsig_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant