Consolidated Balance Sheets Parentheticals - shares shares in Millions  | 
6 Months Ended | 12 Months Ended | 
|---|---|---|
Jun. 30, 2018  | 
Dec. 31, 2017  | 
|
| General Partner Ownership Interest Percentage | 2.00% | 2.00% | 
| General Partners' Capital Account, Units Issued | 9.9 | 9.9 | 
| General partner units outstanding | 9.9 | 9.9 | 
| Common Units [Member] | ||
| Limited Partners' Capital Account, Units Issued | 348.6 | 348.6 | 
| Partnership unitholders units outstanding | 348.6 | 348.6 | 
| Subordinated Units [Member] | ||
| Limited Partners' Capital Account, Units Issued | 135.4 | 135.4 | 
| Partnership unitholders units outstanding | 135.4 | 135.4 | 
| X | ||||||||||
- Definition The number of general partner units issued. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The number of general partner units outstanding. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Percentage investment held by the managing member or general partner of the limited liability company (LLC) or limited partnership (LP). Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The number of limited partner units issued. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The number of limited partner units outstanding. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Details 
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- Details 
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