Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Financial liabilities [Abstract]
Liquidity option agreement		$ 269.6		$ 245.1
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		(45.0)		18.4		$ (30.6)
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives		36.5		3.2
Commodity derivatives		541.4		255.6
Financial assets		577.9		258.8
Financial liabilities [Abstract]
Liquidity option agreement		269.6		245.1
Interest rate derivatives		1.1		3.7
Commodity derivatives		739.4		143.0
Financial liabilities		1,010.1		391.8
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0		0.0
Commodity derivatives		84.5		109.5
Financial assets		84.5		109.5
Financial liabilities [Abstract]
Liquidity option agreement		0.0		0.0
Interest rate derivatives		0.0		0.0
Commodity derivatives		136.8		31.3
Financial liabilities		136.8		31.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives		36.5		3.2
Commodity derivatives		455.2		145.2
Financial assets		491.7		148.4
Financial liabilities [Abstract]
Liquidity option agreement		0.0		0.0
Interest rate derivatives		1.1		3.7
Commodity derivatives		602.3		109.2
Financial liabilities		603.4		112.9
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0		0.0
Commodity derivatives		1.7		0.9
Financial assets		1.7		0.9
Financial liabilities [Abstract]
Liquidity option agreement		269.6		245.1
Interest rate derivatives		0.0		0.0
Commodity derivatives		0.3		2.5
Financial liabilities		269.9		247.6
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Financial liability balance, net, beginning of period		(246.7)	[1]	(219.3)
Total gains (losses) included in:
Settlements		(0.5)		0.1
Transfers out of Level 3	[1]	1.8		18.0
Financial liability balance, net, end of period		(268.2)	[1]	(246.7)	[1]	$ (219.3)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		1.7		(0.9)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]
Total gains (losses) included in:
Other comprehensive income (loss)		(0.5)		(19.2)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]
Total gains (losses) included in:
Net income	[2]	2.2		(0.9)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]
Total gains (losses) included in:
Net income		$ (24.5)		$ (25.4)

[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, 2016 and 2015.

[2] There were $1.7 million of unrealized gains and $0.9 million of unrealized losses included in these amounts for the years ended December 31, 2016 and 2015, respectively.