Capital Accounts, Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 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			(669.6)	184.9
Reclassification of losses (gains) to net income during period			663.0	(305.6)
Total other comprehensive income (loss) for period	$ 26.9	$ 97.8	(6.6)	(120.7)
Accumulated Other Comprehensive Income (Loss), Ending balance	(171.8)	(49.3)	(171.8)	(49.3)
Interest expense	315.9	320.5	954.8	958.2
Revenue	(10,831.3)	(6,922.0)	(29,436.7)	(20,155.5)
Operating costs and expenses	9,408.5	5,571.2	25,028.6	16,001.9
Total	(1,182.1)	(1,084.0)	(3,690.7)	(3,519.8)
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	(207.7)
Reclassification of losses (gains) to net income during period			29.1	29.2
Total other comprehensive income (loss) for period			212.0	(178.5)
Accumulated Other Comprehensive Income (Loss), Ending balance	137.7	(164.6)	137.7	(164.6)
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			(852.2)	392.7
Reclassification of losses (gains) to net income during period			633.8	(334.8)
Total other comprehensive income (loss) for period			(218.4)	57.9
Accumulated Other Comprehensive Income (Loss), Ending balance	(311.6)	113.0	(311.6)	113.0
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			0.1	0.0
Total other comprehensive income (loss) for period			(0.2)	(0.1)
Accumulated Other Comprehensive Income (Loss), Ending balance	2.1	2.3	2.1	2.3
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Total	127.4	39.4	662.9	(305.6)
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	10.3	9.9	29.1	29.2
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member] | Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Revenue	116.8	19.5	614.9	(344.7)
Operating costs and expenses	$ 0.3	$ 10.0	$ 18.9	$ 9.9