Business Segments, Gross Operating Margin (Details) - USD ($) $ 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
Business Segments [Abstract]
Operating income		$ 1,640.4	$ 1,643.3	$ 986.4	$ 1,138.5	$ 1,079.4	$ 879.2	$ 938.7	$ 1,031.6	$ 5,408.6	$ 3,928.9	$ 3,580.7
Adjustments to reconcile operating income to total gross operating margin:
Add depreciation, amortization and accretion expense in operating costs and expenses										1,687.0	1,531.3	1,456.7
Add asset impairment and related charges in operating costs and expenses										50.5	49.8	52.8
Subtract net gains attributable to asset sales in operating costs and expenses										(28.7)	(10.7)	(2.5)
Add general and administrative costs										208.3	181.1	160.1
Adjustments for make-up rights on certain new pipeline projects:
Add non-refundable payments received from shippers attributable to make-up rights	[1]									21.5	24.1	17.5
Subtract the subsequent recognition of revenues attributable to make-up rights	[2]									(56.2)	(29.9)	(34.6)
Total segment gross operating margin										$ 7,291.0	$ 5,674.6	$ 5,230.7

[1] Since make-up rights entail a future performance obligation by the pipeline to the shipper, these receipts are recorded as deferred revenue for GAAP purposes; however, these receipts are included in gross operating margin in the period of receipt since they are nonrefundable to the shipper.

[2] As deferred revenues attributable to make-up rights are subsequently recognized as revenue under GAAP, gross operating margin must be adjusted to remove such amounts to prevent duplication since the associated non-refundable payments were previously included in gross operating margin.