Capital Accounts, Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Abstract]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ 71.4	$ 50.9
Other comprehensive income (loss) for period, before reclassifications	(3.2)	124.9
Reclassification of losses (gains) to net income during period	(233.4)	(104.4)
Total other comprehensive income (loss) for period	(236.6)	20.5	$ 222.6
Accumulated Other Comprehensive Income (Loss), Ending balance	(165.2)	71.4	50.9
Interest expense	1,287.4	1,243.0	1,096.7
Revenue	(27,199.7)	(32,789.2)	(36,534.2)
Operating costs and expenses	22,371.1	27,061.8	31,397.3
Total	(3,885.7)	(4,687.1)	(4,238.5)
Cash Flow Hedges [Member] | Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	55.1	152.7
Other comprehensive income (loss) for period, before reclassifications	124.4	44.1
Reclassification of losses (gains) to net income during period	(272.7)	(141.7)
Total other comprehensive income (loss) for period	(148.3)	(97.6)
Accumulated Other Comprehensive Income (Loss), Ending balance	(93.2)	55.1	152.7
Cash Flow Hedges [Member] | Interest Rate Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	13.9	(104.8)
Other comprehensive income (loss) for period, before reclassifications	(127.5)	81.4
Reclassification of losses (gains) to net income during period	39.3	37.3
Total other comprehensive income (loss) for period	(88.2)	118.7
Accumulated Other Comprehensive Income (Loss), Ending balance	(74.3)	13.9	(104.8)
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	2.4	3.0
Other comprehensive income (loss) for period, before reclassifications	(0.1)	(0.6)
Reclassification of losses (gains) to net income during period	0.0	0.0
Total other comprehensive income (loss) for period	(0.1)	(0.6)
Accumulated Other Comprehensive Income (Loss), Ending balance	2.3	2.4	$ 3.0
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Total	(233.4)	(104.4)
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member] | Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Revenue	(282.6)	(152.4)
Operating costs and expenses	9.9	10.7
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	$ 39.3	$ 37.3