Fair Value (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2011

Dec. 31, 2010

Fair Value [Abstract]

Carrying Amounts And Fair Values Of Our Financial Instruments

Assets (Liabilities)	December 31, 2010						September 30, 2011
	Carrying Amount			Fair Value			Carrying Amount			Fair Value
Cash and cash equivalents	$	7,483		$	7,483		$	198,065		$	198,065
Restricted cash	$	14,379		$	14,379		$	—		$	—
Energy commodity derivatives deposits	$	22,302		$	22,302		$	(7,059	)	$	(7,059	)
Long-term receivables	$	1,167		$	1,161		$	3,862		$	3,830
Energy commodity derivatives contracts (current)	$	(11,790	)	$	(11,790	)	$	36,205		$	36,205
Energy commodity derivatives contracts (noncurrent)	$	(4,920	)	$	(4,920	)	$	1,030		$	1,030
Debt	$	(1,906,148	)	$	(2,048,895	)	$	(2,153,437	)	$	(2,401,380	)

Recurring Fair Value Measurements Of Our Commodity Contracts And Interest Rate Swaps

Assets (Liabilities)			Fair Value Measurements as of September 30, 2011 using:
	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Energy commodity derivatives contracts (current)	$	36,205	$	36,205	$	—	$	—
Energy commodity derivatives contracts (noncurrent)	$	1,030	$	1,030	$	—	$	—

Assets (Liabilities)				Fair Value Measurements as of December 31, 2010 using:
	Total			Quoted Prices in Active Markets for Identical Assets (Level 1)			Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Energy commodity derivatives contracts (current)	$	(11,790	)	$	(11,790	)	$	—	$	—
Energy commodity derivatives contracts (noncurrent)	$	(4,920	)	$	(4,920	)	$	—	$	—