CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Partners' Capital [Abstract]
Common units issued (in shares)	196.5	175.7
Common units outstanding (in shares)	196.5	175.7
Class B units issued (in shares)	22.9	22.9
Class B units outstanding (in shares)	22.9	22.9

The number of limited partner class b units outstanding. The limited partners are partners of a publicly traded limited partnership or master limited partnership. Limited partners have limited liability and do not manage the partnership.

No definition available.

Name:	bwp_LimitedPartnerCapitalAccountClassBUnitsOutstanding
Namespace Prefix:	bwp_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant