Fair Value Measurements (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2012

Dec. 31, 2011

Fair Value Disclosures [Abstract]

Fair value of financial assets and liabilities

The fair value of financial assets and liabilities at September 30, 2012, is shown in the table below:

	Fair Value Measurements
	September 30, 2012
	Level 1		Level 2		Level 3		Netting Adjustments			Total
			(in millions)
Financial Assets
Commodity derivative instruments - short-term	$	—	$	199.4	$	—	$	(12.2	)	$	187.2
Commodity derivative instruments - long-term	—		38.2		—		(3.0		)	$	35.2
Total financial assets	$	—	$	237.6	$	—	$	(15.2	)	$	222.4
Financial Liabilities
Commodity derivative instruments - short-term	$	—	$	12.3	$	—	$	(12.2	)	$	0.1
Interest rate swaps - short-term	—		2.6		—		—			$	2.6
Commodity derivative instruments - long-term	—		3.0		—		(3.0		)	$	—
Interest rate swaps - long-term	—		4.1		—		—			$	4.1
Total financial liabilities	$	—	$	22.0	$	—	$	(15.2	)	$	6.8

The fair value of financial assets and liabilities at December 31, 2011, is shown in the table below:

	Fair Value Measurements
	December 31, 2011
	Level 1		Level 2		Level 3		Netting Adjustments			Total
			(in millions)
Financial Assets
Commodity derivative instruments - short-term	$	—	$	284.1	$	—	$	(10.4	)	$	273.7
Commodity derivative instruments - long-term	—		123.5		—		—			$	123.5
Total financial assets	$	—	$	407.6	$	—	$	(10.4	)	$	397.2
Financial Liabilities
Commodity derivative instruments - short-term	$	—	$	11.7	$	—	$	(10.4	)	$	1.3
Commodity derivative instruments - long-term	—		—		—		—			$	—
Total financial liabilities	$	—	$	11.7	$	—	$	(10.4	)	$	1.3

Change in fair value of Level 3 assets and liabilities

The change in the fair value of Level 3 commodity derivative instruments assets and liabilities for the nine months ended September 30, 2012, is shown below:

	Change in Level 3 Fair Value Measurements
	2012
	(in millions)
Balance at January 1,	$	—
Realized gains and losses	0.6
Unrealized gains and losses	3.8
Settlements	(0.6		)
Transfers out of Level 3	(3.8		)
Balance at September 30,	$	—

Fair value and related carrying amount of certain financial instruments

The following table discloses the fair value and related carrying amount of certain financial instruments not disclosed in other notes to the condensed consolidated financial statements in this quarterly report on Form 10-Q:

	Carrying Amount		Level 1 Fair Value		Carrying Amount		Level 1 Fair Value
	September 30, 2012				December 31, 2011
	(in millions)
Financial liabilities
Checks outstanding in excess of cash balances	$	27.5	$	27.5	$	29.4	$	29.4
Long-term debt	$	3,180.7	$	3,330.4	$	1,679.4	$	1,754.9