Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016	[1]	Dec. 31, 2015	[1]
Quarterly Financial Information Disclosure [Abstract]
Total Revenues	$ 126.8	$ 121.8	$ 112.4	$ 109.1	$ 115.1	$ 107.7	$ 112.9	$ 117.2	$ 470.1		$ 452.9		$ 485.5
Income Before Income Taxes	102.6	99.5	91.7	98.1	95.5	84.4	98.6	99.0	391.9		377.5		373.9
Net income	102.5	99.5	91.7	98.1	95.5	84.4	98.6	99.0	391.8	[2]	377.5	[2],[3]	374.0	[2],[3]
Net Income Attributable to the Partnership	86.4	72.6	65.5	70.8	69.5	56.3	63.8	55.3	295.3		244.9		167.1
Limited Partners' Interest in Net Income Attributable to the Partnership	$ 65.8	$ 55.0	$ 51.2	$ 58.7	$ 59.8	$ 49.1	$ 58.8	$ 52.2	$ 230.7		$ 219.9		$ 162.1
Net income per Limited Partner Unit - Basic and Diluted (in dollars per share)	$ 0.35	$ 0.31	$ 0.29	$ 0.33	$ 0.34	$ 0.28	$ 0.35	$ 0.36

[1] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.

[2] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.

[3] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.