Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended												9 Months Ended
	Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities													$ 5.3		$ 15.9
Level 3 [Member]
Financial assets [Abstract]
Commodity derivatives	2.3												2.3
Financial liabilities [Abstract]
Commodity derivatives	2.2												2.2
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives	31.4												31.4			45.2
Commodity derivatives	69.5												69.5			61.6
Financial assets	100.9												100.9			106.8
Financial liabilities [Abstract]
Interest rate derivatives	10.3												10.3			192.6
Commodity derivatives	51.9												51.9			45.5
Financial liabilities	62.2												62.2			238.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives	0												0			0
Commodity derivatives	34.0												34.0			11.4
Financial assets	34.0												34.0			11.4
Financial liabilities [Abstract]
Interest rate derivatives	0												0			0
Commodity derivatives	19.5												19.5			13.1
Financial liabilities	19.5												19.5			13.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives	31.4												31.4			45.2
Commodity derivatives	33.2												33.2			47.8
Financial assets	64.6												64.6			93.0
Financial liabilities [Abstract]
Interest rate derivatives	10.3												10.3			192.6
Commodity derivatives	30.2												30.2			28.5
Financial liabilities	40.5												40.5			221.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives	0												0			0
Commodity derivatives	2.3												2.3			2.4
Financial assets	2.3												2.3			2.4
Financial liabilities [Abstract]
Interest rate derivatives	0												0			0
Commodity derivatives	2.2												2.2			3.9
Financial liabilities	2.2												2.2			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	(0.2)		(0.6)		(1.5)		4.9		0.9		0.4		(1.5)		0.4
Total gains (losses) included in:
Net income	1.1	[1]	(0.2)	[1]	(0.6)	[1]	(0.6)	[1]	(1.3)	[1]	0.5	[1]
Other comprehensive income (loss)	(0.9)		0		0		3.5		6.0		0.5
Settlements	0.1		0.6		1.5		1.4		(0.7)		(0.5)
Financial asset (liability) balance, net, end of period	0.1	[2]	(0.2)		(0.6)		9.2		4.9		0.9		0.1	[2]	9.2
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities	$ 1.1						$ 0.8						$ 2.4		$ (1.1)

[1] There were unrealized gains of $1.1 million and $2.4 million included in these amounts for the three and nine months ended September 30, 2013, respectively. There were $0.8 million of unrealized gains and $1.1 million of unrealized losses included in these amounts for the three and nine months ended September 30, 2012, respectively.

[2] There were no transfers into or out of Level 3 during the three or nine months ended September 30, 2013.