Fair Value Measurements Fair Value Measurements Balance Sheet Location (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 4,156	$ 5,332	$ 1,490	$ 11,623
Derivative Liability, Fair Value, Gross Liability	95,581	131,010	131,229	143,887
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Net	1,805	3,384	3,796
Derivative Liability, Fair Value, Net	20,119	26,793	27,779
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Net	0	1,948	421
Derivative Liability, Fair Value, Net	73,111	104,218	106,177
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Derivative Assets, Current [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	846	2,874	3,263
Derivative Asset, Fair Value, Gross Liability	0	0	0
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Derivative Assets, Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	959	510	533
Derivative Asset, Fair Value, Gross Liability	0	0	0
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Derivative Liabilities, Current [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	0	0	0
Derivative Liability, Fair Value, Gross Liability	1,317	1,993	1,534
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Derivative Liabilities, Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	0	0	0
Derivative Liability, Fair Value, Gross Liability	375	821	555
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Derivative Assets, Current [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	362	421
Derivative Asset, Fair Value, Gross Liability	0	0	3,361
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Derivative Assets, Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	0	0
Derivative Asset, Fair Value, Gross Liability	0	0	(634)
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Derivative Liabilities, Current [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	0	1,180	0
Derivative Liability, Fair Value, Gross Liability	1,795	4,957	33
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Derivative Liabilities, Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	0	406	0
Derivative Liability, Fair Value, Gross Liability	6,601	5,153	4,527
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Derivative Liabilities, Current [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	0	0	0
Derivative Liability, Fair Value, Gross Liability	7,039	7,038	7,029
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Derivative Liabilities, Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	0	0	0
Derivative Liability, Fair Value, Gross Liability	11,388	16,941	18,661
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member] | Derivative Liabilities, Current [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	0	0	0
Derivative Liability, Fair Value, Gross Liability	64,715	94,108	77,913
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member] | Derivative Liabilities, Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	0	0	0
Derivative Liability, Fair Value, Gross Liability	$ 0	$ 0	$ 20,977