Revenues, Impact of Change in Accounting Policy (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017		Dec. 31, 2017
Assets [Abstract]
Accounts receivable - trade, net	$ 4,439.9				$ 4,358.4
Prepaid and other current assets	353.3				312.7
Property, plant and equipment, net	36,416.3				35,620.4
Other assets	210.0				196.4
Liabilities and Equity
Other long-term liabilities	603.4				578.4
Partners' equity	22,753.3				$ 22,547.2
Condensed Consolidated Statement of Operations Information [Abstract]
Revenues	9,298.5	[1]	$ 7,320.4	[2]
Costs and Expenses [Abstract]
Operating costs and expenses	8,222.7		6,333.2
Operating Activities [Abstract]
Net income	911.5		771.0
Net effect of changes in operating accounts	(203.1)		(288.8)
Investing Activities [Abstract]
Contributions in aid of construction costs	0.0		$ 15.6
Balances Without Adoption [Member] | ASC 606 [Member]
Assets [Abstract]
Accounts receivable - trade, net	4,504.5
Prepaid and other current assets	288.9
Property, plant and equipment, net	36,410.9
Other assets	209.8
Liabilities and Equity
Other long-term liabilities	601.2
Partners' equity	22,750.1
Condensed Consolidated Statement of Operations Information [Abstract]
Revenues	9,181.5
Costs and Expenses [Abstract]
Operating costs and expenses	8,108.9
Operating Activities [Abstract]
Net income	908.3
Net effect of changes in operating accounts	(205.3)
Investing Activities [Abstract]
Contributions in aid of construction costs	5.4
Impact of Adoption [Member] | ASC 606 [Member]
Assets [Abstract]
Accounts receivable - trade, net	(64.6)
Prepaid and other current assets	64.4
Property, plant and equipment, net	5.4
Other assets	0.2
Liabilities and Equity
Other long-term liabilities	2.2
Partners' equity	3.2
Condensed Consolidated Statement of Operations Information [Abstract]
Revenues	117.0
Costs and Expenses [Abstract]
Operating costs and expenses	113.8
Operating Activities [Abstract]
Net income	3.2
Net effect of changes in operating accounts	2.2
Investing Activities [Abstract]
Contributions in aid of construction costs	$ (5.4)

[1] Revenues are accounted for under ASC 606 upon implementation at January 1, 2018.

[2] Revenues are accounted for under ASC 605 for historical periods prior to January 1, 2018.