Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total Revenues	$ 142.3	$ 153.5	$ 129.3	$ 99.6	$ 126.8	$ 121.8	$ 112.4	$ 109.1	$ 524.7	$ 470.1	$ 452.9
Income Before Income Taxes	148.2	154.3	115.5	64.8	102.6	99.5	91.7	98.1	482.8	391.9	377.5
Net income	148.0	154.2	115.4	64.8	102.5	99.5	91.7	98.1	482.4	391.8	377.5
Net Income Attributable to the Partnership	141.1	148.3	110.7	64.0	86.4	72.6	65.5	70.8	464.1	295.3	244.9
Limited Partners’ Interest in Net Income Attributable to the Partnership	$ 101.3	$ 112.3	$ 79.1	$ 37.0	$ 65.8	$ 55.0	$ 51.2	$ 58.7	$ 329.7	$ 230.7	$ 219.9
Net income per Limited Partner Unit - Basic and Diluted (in dollars per share)	$ 0.45	$ 0.50	$ 0.35	$ 0.18	$ 0.35	$ 0.31	$ 0.29	$ 0.33