Capital Accounts, Issuances of Equity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2021	Jun. 30, 2020
Net Cash Proceeds from Sale of Common Units [Abstract]
Common units issued in connection with the vesting of phantom unit awards, net (in units)	203,066	3,553,313
Buyback Program:
Total of common units repurchased under a buyback program (in units)		709,816
Common units acquired in connection with buyback program			$ 13.9	$ 140.1
Long-Term Incentive Plan (2008) [Member]
Net Cash Proceeds from Sale of Common Units [Abstract]
Common units issued in connection with the vesting of phantom unit awards, net (in units)			3,756,379
2019 Buyback Program [Member]
Buyback Program:
Amount authorized under 2019 Buyback Program	$ 2,000.0		$ 2,000.0
Remaining available capacity under the 2019 Buyback Program	1,720.0		$ 1,720.0
Total of common units repurchased under a buyback program (in units)			709,816	6,357,739
Common units acquired in connection with buyback program			$ 13.9	$ 140.1
At-the-Market Registration [Member]
Registration Statements and Equity Offerings [Line Items]
Maximum common units authorized for issuance	2,540.0		2,540.0
Remaining units available for issuance	$ 2,540.0		2,540.0
Employee Unit Purchase Plan [Member]
Net Cash Proceeds from Sale of Common Units [Abstract]
Employer contribution to EUPP			$ 2.0
Distribution Reinvestment and Employee Unit Purchase Plans [Member]
Net Cash Proceeds from Sale of Common Units [Abstract]
Number of common units purchased on the open market and delivered to participants (in units)			3,166,524