Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		$ (136.9)	$ 20.3
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives		9.5		$ 0.1
Value before application of CME Rule 814		127.8		234.9
Impact of CME Rule 814 change		(42.5)		(73.2)
Total commodity derivatives		85.3		161.7
Total		94.8		161.8
Financial liabilities [Abstract]
Liquidity option agreement		341.4		333.9
Interest rate derivatives		1.6		1.7
Commodity derivatives:
Value before application of CME Rule 814		340.6		390.7
Impact of CME Rule 814 change		(157.6)		(213.8)
Total commodity derivatives		183.0		176.9
Total		526.0		512.5
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0		0.0
Value before application of CME Rule 814		29.5		47.1
Impact of CME Rule 814 change		(27.0)		(47.1)
Total commodity derivatives		2.5		0.0
Total		2.5		0.0
Financial liabilities [Abstract]
Liquidity option agreement		0.0		0.0
Interest rate derivatives		0.0		0.0
Commodity derivatives:
Value before application of CME Rule 814		64.3		118.4
Impact of CME Rule 814 change		(41.3)		(118.4)
Total commodity derivatives		23.0		0.0
Total		23.0		0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives		9.5		0.1
Value before application of CME Rule 814		97.9		184.9
Impact of CME Rule 814 change		(15.5)		(26.1)
Total commodity derivatives		82.4		158.8
Total		91.9		158.9
Financial liabilities [Abstract]
Liquidity option agreement		0.0		0.0
Interest rate derivatives		1.6		1.7
Commodity derivatives:
Value before application of CME Rule 814		275.4		270.6
Impact of CME Rule 814 change		(116.3)		(95.4)
Total commodity derivatives		159.1		175.2
Total		160.7		176.9
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0		0.0
Value before application of CME Rule 814		0.4		2.9
Impact of CME Rule 814 change		0.0		0.0
Total commodity derivatives		0.4		2.9
Total		0.4		2.9
Financial liabilities [Abstract]
Liquidity option agreement		341.4		333.9
Interest rate derivatives		0.0		0.0
Commodity derivatives:
Value before application of CME Rule 814		0.9		1.7
Impact of CME Rule 814 change		0.0		0.0
Total commodity derivatives		0.9		1.7
Total		342.3		$ 335.6
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Financial liability balance, net, beginning of period		(332.7)	(268.2)
Total gains (losses) included in:
Transfers out of Level 3		0.0	0.0
Financial liability balance, net, end of period		(341.9)	(274.4)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		(1.7)	(0.7)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]
Total gains (losses) included in:
Net income	[1]	(0.5)	0.7
Settlements	[1]	(1.2)	(1.4)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]
Total gains (losses) included in:
Net income		(7.5)	(5.5)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]
Total gains (losses) included in:
Other comprehensive income		$ 0.0	$ 0.0

[1] There were unrealized losses of $1.7 million and $0.7 million included in these amounts for the three months ended March 31, 2018 and 2017, respectively.