Derivative Instruments, Hedging Activities and Fair Value Measurements, Gains and Losses on Derivative Instruments and Related Hedged Items (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gains (losses)		$ (12.5)	$ (322.1)	$ 83.8	$ (458.9)
Derivatives in fair value hedging relationships [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative		6.9	5.4	(1.6)	5.9
Gain (Loss) Recognized in Income on Hedged Item		(3.6)	(5.5)	6.3	(3.2)
Derivatives in fair value hedging relationships [Member] | Interest rate derivatives [Member] | Interest expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative		0.0	0.6	0.0	1.3
Gain (Loss) Recognized in Income on Hedged Item		0.0	(0.6)	0.0	(1.4)
Derivatives in fair value hedging relationships [Member] | Commodity derivatives [Member] | Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative		6.9	4.8	(1.6)	4.6
Gain (Loss) Recognized in Income on Hedged Item		(3.6)	(4.9)	6.3	(1.8)
Derivatives in cash flow hedging relationships [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Value Recognized in Other Comprehensive Income (Loss) on Derivative		76.3	(10.1)	(18.9)	4.4
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income (Loss) to Income		(7.0)	(48.6)	42.1	(44.6)
Derivatives in cash flow hedging relationships [Member] | Interest rate derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Value Recognized in Other Comprehensive Income (Loss) on Derivative		(5.2)	3.5	(5.2)	14.6
Accumulated other comprehensive loss related to interest rate derivative instruments expected to be reclassified to earnings in interest expense over the next twelve months		(38.4)		(38.4)
Derivatives in cash flow hedging relationships [Member] | Interest rate derivatives [Member] | Interest expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income (Loss) to Income		(9.2)	(9.4)	(18.4)	(19.9)
Derivatives in cash flow hedging relationships [Member] | Commodity derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net accumulated other comprehensive income (loss) related to commodity derivative instruments expected to be reclassified to earnings over the next twelve months		85.9		85.9
Accumulated other comprehensive income (loss) related to commodity derivative instruments expected to be reclassified to revenue over the next twelve months		88.6		88.6
Accumulated other comprehensive income (loss) related to commodity derivative instruments expected to be reclassified to operating costs and expenses over the next twelve months		(2.7)		(2.7)
Derivatives in cash flow hedging relationships [Member] | Commodity derivatives [Member] | Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Value Recognized in Other Comprehensive Income (Loss) on Derivative	[1]	84.3	(14.2)	(2.4)	(11.2)
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income (Loss) to Income		2.5	(39.4)	67.8	(25.4)
Derivatives in cash flow hedging relationships [Member] | Commodity derivatives [Member] | Operating costs and expenses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Value Recognized in Other Comprehensive Income (Loss) on Derivative	[1]	(2.8)	0.6	(11.3)	1.0
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income (Loss) to Income		(0.3)	0.2	(7.3)	0.7
Derivatives not designated as hedging instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative		(25.0)	(406.3)	70.2	(561.3)
Realized gains (losses)				(11.7)
Unrealized gains (losses)				81.9
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative		(20.2)	(406.3)	74.9	(559.8)
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Operating costs and expenses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative		$ (4.8)	$ 0.0	(4.7)	$ (1.5)
Derivatives in fair value hedging relationships and not designated as hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gains (losses)				$ 84.7

[1] The fair value of these derivative instruments will be reclassified to their respective locations on the Unaudited Condensed Statement of Consolidated Operations upon settlement of the underlying derivative transactions, as appropriate.