Commitments and Contingencies Letter of Credit Proceeds (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Letter of Credit Proceeds	$ 37.7	$ 27.7
Letter of Credit Proceeds Recognized into Transportation Revenue	$ 34.4	$ 26.2

Amount of Letter of Credit proceeds recognized into transportation revenue.

No definition available.

Name:	bwp_LetterofCreditProceedsRecognizedintoTransportationRevenue
Namespace Prefix:	bwp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration