Partners' Capital (Schedule of Partnership Distributions) (Details) - USD ($) $ / shares in Units, $ in Millions																											12 Months Ended
	Feb. 14, 2018	Feb. 07, 2018	Nov. 14, 2017	Nov. 07, 2017	Aug. 14, 2017	Aug. 07, 2017	May 15, 2017	May 08, 2017	Feb. 14, 2017	Feb. 07, 2017	Nov. 14, 2016	Nov. 07, 2016	Aug. 12, 2016	Aug. 05, 2016	May 13, 2016	May 06, 2016	Feb. 12, 2016	Feb. 05, 2016	Nov. 13, 2015	Nov. 06, 2015	Aug. 14, 2015	Aug. 07, 2015	May 15, 2015	May 08, 2015	Feb. 13, 2015	Feb. 06, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Partners Capital Summary Of Quarterly Distributions Of Available Cash [Abstract]
Distribution to limited partner, record date				Nov. 07, 2017		Aug. 07, 2017		May 08, 2017		Feb. 07, 2017		Nov. 07, 2016		Aug. 05, 2016		May 06, 2016		Feb. 05, 2016		Nov. 06, 2015		Aug. 07, 2015		May 08, 2015		Feb. 06, 2015
Distribution to limited partner, distribution date			Nov. 14, 2017		Aug. 14, 2017		May 15, 2017		Feb. 14, 2017		Nov. 14, 2016		Aug. 12, 2016		May 13, 2016		Feb. 12, 2016		Nov. 13, 2015		Aug. 14, 2015		May 15, 2015		Feb. 13, 2015
Distribution Made to Limited Partner, Distributions Paid, Per Unit			$ 0.60		$ 0.60		$ 0.60		$ 0.60		$ 0.60		$ 0.60		$ 0.60		$ 1.3750		$ 1.3750		$ 1.3750		$ 1.3750		$ 1.3750
Distribution amount			$ 42.2		$ 41.8		$ 41.8		$ 41.8		$ 41.4		$ 41.4		$ 41.4		$ 95.6		$ 94.3		$ 25.7		$ 25.7		$ 25.8		$ 167.6	$ 219.8	$ 171.5
Cash distribution | Subsequent Event
Disclosure Partners Capital Summary Of Quarterly Distributions Of Available Cash [Abstract]
Distribution to limited partner, record date		Feb. 07, 2018
Distribution to limited partner, distribution date	Feb. 14, 2018
Distribution declared per limited partner unit	$ 0.600