Partners' Capital (Schedule of Partners' Capital Account, Distributions) (Details) - USD ($) $ / shares in Units, $ in Millions									6 Months Ended
May 15, 2020	May 08, 2020	Feb. 14, 2020	Feb. 07, 2020	May 15, 2019	May 08, 2019	Feb. 14, 2019	Feb. 07, 2019	Jun. 30, 2020	Jun. 30, 2019
Distribution Made to Limited Partner [Line Items]
Distribution Made to Limited Partner, Date of Record		May 08, 2020		Feb. 07, 2020		May 08, 2019		Feb. 07, 2019
Distribution Made to Limited Partner, Distribution Date	May 15, 2020		Feb. 14, 2020		May 15, 2019		Feb. 14, 2019
Distribution Made to Member or Limited Partner, Distributions Paid, Per Unit	$ 0.625		$ 0.625		$ 0.60		$ 0.60
Distribution Made to Limited Partner, Cash Distributions Paid	$ 45.7		$ 45.3		$ 43.1		$ 43.1		$ 91.0	$ 86.2

Amount of cash distribution paid to unit-holder of limited partnership (LP).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

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Date of record for distribution of cash or unit to limited partner of limited partnership (LP), in CCYY-MM-DD format.

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Date of distribution for distribution of cash or unit to limited partner of limited partnership (LP), in CCYY-MM-DD format.

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Per unit of ownership amount of cash distributions paid to unit-holder of a limited partnership (LP).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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