Capital Accounts, Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 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	(278.5)	196.9
Reclassification of losses (gains) to net income during period	624.7	(146.0)
Total other comprehensive income (loss) for period	346.2	50.9
Accumulated Other Comprehensive Income (Loss), Ending balance	181.0	122.3
Interest expense	322.8	317.5
Revenue	(9,155.3)	(7,482.5)
Operating costs and expenses	7,553.4	6,060.3
Total	(1,362.6)	(1,375.0)
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	(278.1)
Reclassification of losses (gains) to net income during period	8.6	9.6
Total other comprehensive income (loss) for period	191.5	(268.5)
Accumulated Other Comprehensive Income (Loss), Ending balance	117.2	(254.6)
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	(461.2)	475.1
Reclassification of losses (gains) to net income during period	616.1	(155.6)
Total other comprehensive income (loss) for period	154.9	319.5
Accumulated Other Comprehensive Income (Loss), Ending balance	61.7	374.6
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	(0.2)	(0.1)
Reclassification of losses (gains) to net income during period	0.0	0.0
Total other comprehensive income (loss) for period	(0.2)	(0.1)
Accumulated Other Comprehensive Income (Loss), Ending balance	2.1	2.3
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Total	624.7	(146.0)
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	8.6	9.6
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member] | Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Revenue	597.4	(154.4)
Operating costs and expenses	$ 18.7	$ (1.2)