Capital Accounts, Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
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			184.9	34.2
Reclassification of losses (gains) to net income during period			(305.6)	(124.2)
Total other comprehensive income (loss) for period	$ 97.8	$ (28.4)	(120.7)	(90.0)
Accumulated Other Comprehensive Income (Loss), Ending balance	(49.3)	(39.1)	(49.3)	(39.1)
Interest expense	320.5	382.9	958.2	950.2
Revenue	(6,922.0)	(7,964.1)	(20,155.5)	(24,783.9)
Operating costs and expenses	5,571.2	6,573.7	16,001.9	20,394.3
Total	(1,084.0)	(1,044.8)	(3,519.8)	(3,561.7)
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			(207.7)	(23.8)
Reclassification of losses (gains) to net income during period			29.2	27.8
Total other comprehensive income (loss) for period			(178.5)	4.0
Accumulated Other Comprehensive Income (Loss), Ending balance	(164.6)	(100.8)	(164.6)	(100.8)
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			392.7	58.6
Reclassification of losses (gains) to net income during period			(334.8)	(152.0)
Total other comprehensive income (loss) for period			57.9	(93.4)
Accumulated Other Comprehensive Income (Loss), Ending balance	113.0	59.3	113.0	59.3
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	2.3	2.4
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Total	39.4	(82.1)	(305.6)	(124.2)
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	9.9	9.4	29.2	27.8
Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member] | Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Abstract]
Revenue	19.5	(93.6)	(344.7)	(161.4)
Operating costs and expenses	$ 10.0	$ 2.1	$ 9.9	$ 9.4