Partners' Capital, Equity and Incentive Compensation Plans (Schedule of Quarterly Cash Dividends) (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
Partners' Capital, Equity and Incentive Compensation Plans [Abstract]
Record Date		Jun. 16, 2017	Mar. 16, 2017	Dec. 16, 2016	Sep. 20, 2016	Jun. 17, 2016	Mar. 18, 2016	Dec. 17, 2015	Sep. 22, 2017
Payment Date		Jul. 06, 2017	Apr. 05, 2017	Jan. 05, 2017	Oct. 05, 2016	Jul. 06, 2016	Apr. 05, 2016	Jan. 06, 2016	Oct. 05, 2017
Per Common Share and Per Unit Rate	$ 0.39	$ 0.39	$ 0.39	$ 0.36	$ 0.36	$ 0.36	$ 0.36	$ 0.32	$ 1.53	$ 1.40
Dividend/Distribution Amount	$ 22.2	$ 21.6	$ 21.4	$ 19.7	$ 19.7	$ 19.7	$ 17.4	$ 15.4	$ 84.9	$ 72.2