Capital Accounts, Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
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	(495.1)	(3.2)
Reclassification of losses (gains) to net income during period	946.2	(233.4)
Total other comprehensive income (loss) for period	451.1	(236.6)	$ 20.5
Accumulated Other Comprehensive Income (Loss), Ending balance	285.9	(165.2)	71.4
Interest expense	1,283.0	1,287.4	1,243.0
Revenue	(40,806.9)	(27,199.7)	(32,789.2)
Operating costs and expenses	35,077.3	22,371.1	27,061.8
Total	(4,755.3)	(3,885.7)	(4,687.1)
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	(677.7)	124.4
Reclassification of losses (gains) to net income during period	907.8	(272.7)
Total other comprehensive income (loss) for period	230.1	(148.3)
Accumulated Other Comprehensive Income (Loss), Ending balance	136.9	(93.2)	55.1
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	(127.5)
Reclassification of losses (gains) to net income during period	38.3	39.3
Total other comprehensive income (loss) for period	221.2	(88.2)
Accumulated Other Comprehensive Income (Loss), Ending balance	146.9	(74.3)	13.9
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	$ 2.4
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Total	946.1	(233.4)
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member] | Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Revenue	893.3	(282.6)
Operating costs and expenses	14.5	9.9
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	$ 38.3	$ 39.3