Capital Accounts, Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Abstract]
Accumulated Other Comprehensive Income (Loss), Beginning Balance			$ (165.2)	$ 71.4
Other comprehensive income (loss) for period, before reclassifications			(569.0)	126.5
Reclassification of losses (gains) to net income during period			535.5	(345.0)
Total other comprehensive income (loss) for period	$ (379.7)	$ (269.4)	(33.5)	(218.5)
Accumulated Other Comprehensive Income (Loss), Ending balance	(198.7)	(147.1)	(198.7)	(147.1)
Interest expense	316.1	320.2	638.9	637.7
Revenue	(9,450.1)	(5,751.0)	(18,605.4)	(13,233.5)
Operating costs and expenses	8,066.7	4,370.4	15,620.1	10,430.7
Total	(1,146.0)	(1,060.8)	(2,508.6)	(2,435.8)
Cash Flow Hedges [Member] | Interest Rate Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Accumulated Other Comprehensive Income (Loss), Beginning Balance			(74.3)	13.9
Other comprehensive income (loss) for period, before reclassifications			182.9	(270.3)
Reclassification of losses (gains) to net income during period			18.8	19.3
Total other comprehensive income (loss) for period			201.7	(251.0)
Accumulated Other Comprehensive Income (Loss), Ending balance	127.4	(237.1)	127.4	(237.1)
Cash Flow Hedges [Member] | Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Accumulated Other Comprehensive Income (Loss), Beginning Balance			(93.2)	55.1
Other comprehensive income (loss) for period, before reclassifications			(751.6)	396.9
Reclassification of losses (gains) to net income during period			516.7	(364.3)
Total other comprehensive income (loss) for period			(234.9)	32.6
Accumulated Other Comprehensive Income (Loss), Ending balance	(328.1)	87.7	(328.1)	87.7
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Accumulated Other Comprehensive Income (Loss), Beginning Balance			2.3	2.4
Other comprehensive income (loss) for period, before reclassifications			(0.3)	(0.1)
Reclassification of losses (gains) to net income during period			0.0	0.0
Total other comprehensive income (loss) for period			(0.3)	(0.1)
Accumulated Other Comprehensive Income (Loss), Ending balance	2.0	2.3	2.0	2.3
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Total	(89.2)	(199.0)	535.5	(345.0)
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member] | Interest Rate Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Interest expense	10.2	9.7	18.8	19.3
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member] | Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Revenue	(99.3)	(209.8)	498.1	(364.2)
Operating costs and expenses	$ (0.1)	$ 1.1	$ 18.6	$ (0.1)