Business Segments (Details) $ in Millions		3 Months Ended
		Mar. 31, 2018 USD ($) Segment	Mar. 31, 2017 USD ($)
Business Segments [Abstract]
Number of reportable segments | Segment		4
Segment Gross Operating Margin [Abstract]
Operating income		$ 1,138.5	$ 1,031.6
Adjustments to reconcile operating income to total gross operating margin:
Add depreciation, amortization and accretion expense in operating costs and expenses		394.3	376.2
Add asset impairment and related charges in operating costs and expenses		0.9	11.2
Add net gains attributable to asset sales in operating costs and expenses		(0.5)	(0.3)
Add general and administrative costs		53.0	50.4
Adjustments for make-up rights on certain new pipeline projects:
Add non-refundable payments received from shippers attributable to make-up rights	[1]	2.7	13.3
Subtract the subsequent recognition of revenues attributable to make-up rights	[2]	(14.2)	(9.1)
Total segment gross operating margin		$ 1,574.7	$ 1,473.3

[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.