Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Financial liabilities [Abstract]
Liquidity option agreement		$ 390.0		$ 333.9
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		(17.8)		(22.8)		$ (45.0)
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives				0.1
Value before application of CME Rule 814		456.9		234.9
Impact of CME Rule 814 change		(295.0)		(73.2)
Total commodity derivatives		161.9		161.7
Financial assets		161.9		161.8
Financial liabilities [Abstract]
Liquidity option agreement		390.0		333.9
Interest rate derivatives				1.7
Commodity derivatives:
Value before application of CME Rule 814		398.1		390.7
Impact of CME Rule 814 change		(235.7)		(213.8)
Total commodity derivatives		162.4		176.9
Financial liabilities		552.4		512.5
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives				0.0
Value before application of CME Rule 814		172.3		47.1
Impact of CME Rule 814 change		(134.8)		(47.1)
Total commodity derivatives		37.5		0.0
Financial assets		37.5		0.0
Financial liabilities [Abstract]
Liquidity option agreement		0.0		0.0
Interest rate derivatives				0.0
Commodity derivatives:
Value before application of CME Rule 814		85.5		118.4
Impact of CME Rule 814 change		(48.6)		(118.4)
Total commodity derivatives		36.9		0.0
Financial liabilities		36.9		0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives				0.1
Value before application of CME Rule 814		282.4		184.9
Impact of CME Rule 814 change		(159.3)		(26.1)
Total commodity derivatives		123.1		158.8
Financial assets		123.1		158.9
Financial liabilities [Abstract]
Liquidity option agreement		0.0		0.0
Interest rate derivatives				1.7
Commodity derivatives:
Value before application of CME Rule 814		291.2		270.6
Impact of CME Rule 814 change		(172.9)		(95.4)
Total commodity derivatives		118.3		175.2
Financial liabilities		118.3		176.9
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives				0.0
Value before application of CME Rule 814		2.2		2.9
Impact of CME Rule 814 change		(0.9)		0.0
Total commodity derivatives		1.3		2.9
Financial assets		1.3		2.9
Financial liabilities [Abstract]
Liquidity option agreement		390.0		333.9
Interest rate derivatives				0.0
Commodity derivatives:
Value before application of CME Rule 814		21.4		1.7
Impact of CME Rule 814 change		(14.2)		0.0
Total commodity derivatives		7.2		1.7
Financial liabilities		397.2		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)	[1]	(268.2)
Total gains (losses) included in:
Transfers out of Level 3	[1]	(2.7)		(0.2)
Financial liability balance, net, end of period		(395.9)	[1]	(332.7)	[1]	$ (268.2)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		(1.2)		(0.1)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]
Total gains (losses) included in:
Other comprehensive income (loss)		(3.2)		0.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]
Total gains (losses) included in:
Net income	[2]	0.7		2.3
Settlements	[2]	(1.9)		(2.4)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]
Total gains (losses) included in:
Net income		$ (56.1)		$ (64.3)

[1] Transfers out of Level 3 into Level 2 were due to shorter remaining transaction maturities falling inside of the Level 2 range at December 31, 2018 and 2017.

[2] There were $1.2 million and $0.1 million of unrealized losses included in these amounts for the years ended December 31, 2018 and 2017, respectively.