Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Financial liabilities [Abstract]
Liquidity option agreement		$ 333.9		$ 269.6
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		(22.8)		(45.0)		$ 18.4
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives		0.1		36.5
Value before application of CME Rule 814		234.9
Impact of CME Rule 814 change		(73.2)
Total commodity derivatives		161.7		541.4
Financial assets		161.8		577.9
Financial liabilities [Abstract]
Liquidity option agreement		333.9		269.6
Interest rate derivatives		1.7		1.1
Commodity derivatives:
Value before application of CME Rule 814		390.7
Impact of CME Rule 814 change		(213.8)
Total commodity derivatives		176.9		739.4
Financial liabilities		512.5		1,010.1
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		47.1
Impact of CME Rule 814 change		(47.1)
Total commodity derivatives		0.0		84.5
Financial assets		0.0		84.5
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		118.4
Impact of CME Rule 814 change		(118.4)
Total commodity derivatives		0.0		136.8
Financial liabilities		0.0		136.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.1		36.5
Value before application of CME Rule 814		184.9
Impact of CME Rule 814 change		(26.1)
Total commodity derivatives		158.8		455.2
Financial assets		158.9		491.7
Financial liabilities [Abstract]
Liquidity option agreement		0.0		0.0
Interest rate derivatives		1.7		1.1
Commodity derivatives:
Value before application of CME Rule 814		270.6
Impact of CME Rule 814 change		(95.4)
Total commodity derivatives		175.2		602.3
Financial liabilities		176.9		603.4
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		2.9
Impact of CME Rule 814 change		0.0
Total commodity derivatives		2.9		1.7
Financial assets		2.9		1.7
Financial liabilities [Abstract]
Liquidity option agreement		333.9		269.6
Interest rate derivatives		0.0		0.0
Commodity derivatives:
Value before application of CME Rule 814		1.7
Impact of CME Rule 814 change		0.0
Total commodity derivatives		1.7		0.3
Financial liabilities		335.6		269.9
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Financial liability balance, net, beginning of period		(268.2)	[1]	(246.7)
Total gains (losses) included in:
Transfers out of Level 3	[1]	(0.2)		1.8
Financial liability balance, net, end of period		(332.7)	[1]	(268.2)	[1]	$ (246.7)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		(0.1)		1.7
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]
Total gains (losses) included in:
Other comprehensive income (loss)		0.1		(0.5)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]
Total gains (losses) included in:
Net income	[2]	2.3		2.2
Settlements	[2]	(2.4)		(0.5)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]
Total gains (losses) included in:
Net income		$ (64.3)		$ (24.5)

[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, 2017 and 2016.

[2] There were $0.1 million of unrealized losses and $1.7 million of unrealized gains included in these amounts for the years ended December 31, 2017 and 2016, respectively.