Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities	$ (1.4)		$ 29.5		$ 25.7
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives	32.6		45.2
Commodity derivatives	41.3		61.6
Financial assets	73.9		106.8
Financial liabilities [Abstract]
Interest rate derivatives	7.8		192.6
Commodity derivatives	55.1		45.5
Financial liabilities	62.9		238.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives	0		0
Commodity derivatives	17.2		11.4
Financial assets	17.2		11.4
Financial liabilities [Abstract]
Interest rate derivatives	0		0
Commodity derivatives	30.8		13.1
Financial liabilities	30.8		13.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives	32.6		45.2
Commodity derivatives	20.2		47.8
Financial assets	52.8		93.0
Financial liabilities [Abstract]
Interest rate derivatives	7.8		192.6
Commodity derivatives	23.6		28.5
Financial liabilities	31.4		221.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives	0		0
Commodity derivatives	3.9		2.4
Financial assets	3.9		2.4
Financial liabilities [Abstract]
Interest rate derivatives	0		0
Commodity derivatives	0.7		3.9
Financial liabilities	0.7		3.9
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Financial asset (liability) balance, net, beginning of period	(1.5)		0.4
Total gains (losses) included in:
Net income	2.8	[1]	(2.9)	[1]
Other comprehensive income (loss)	(0.9)		10.1
Settlements	1.6		0.8
Transfers out of Level 3	1.2	[2]	(9.9)	[2]
Financial asset (liability) balance, net, end of period	3.2		(1.5)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities	$ 4.4		$ 1.9

[1] There were $4.4 million and $1.9 million of unrealized gains included in these amounts for the years ended December 31, 2013 and 2012, respectively.

[2] 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, 2013 and 2012, respectively.