Long Term Debt (Details Textual) (USD $)	9 Months Ended			9 Months Ended			9 Months Ended			9 Months Ended					9 Months Ended							9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 8.125% Senior Unsecured Notes due 2016 [Member]	Sep. 30, 2010 8.125% Senior Unsecured Notes due 2016 [Member]	Dec. 31, 2010 8.125% Senior Unsecured Notes due 2016 [Member]	Sep. 30, 2011 7.75% Senior Unsecured Notes due 2018 [Member]	Sep. 30, 2010 7.75% Senior Unsecured Notes due 2018 [Member]	Dec. 31, 2010 7.75% Senior Unsecured Notes due 2018 [Member]	Sep. 30, 2011 Credit Facility [Member]	Sep. 30, 2010 Credit Facility [Member]	Sep. 30, 2011 Letter of Credit [Member]	Jun. 10, 2011 Letter of Credit [Member]	Jun. 10, 2011 Swing Line Loan [Member]	Sep. 30, 2011 Amended Credit Agreement [Member]	Sep. 30, 2011 Senior Notes [Member]	Sep. 30, 2011 7.125% senior unsecured notes due 2021 [Member]	Apr. 05, 2011 7.125% senior unsecured notes due 2021 [Member]	Dec. 31, 2010 7.125% senior unsecured notes due 2021 [Member]	Jun. 10, 2011 Existing Borrowing Capacity [Member]	Jun. 10, 2011 New Borrowing Capacity [Member]	Sep. 30, 2011 Maximum [Member]	Sep. 30, 2011 Minimum [Member]
Additional Long Term Debt (Textual) [Abstract]
Senior secured revolving credit facility													$ 100,000,000	$ 75,000,000						$ 550,000,000	$ 700,000,000
Interest and financing cost				7,664,000	20,852,000		14,912,000	14,912,000		5,468,000	4,709,000						12,937,000
Debt instrument, covenant description																Ratio of EBITDA to fixed charges is at least 1.75x
Aggregate principal amount outstanding	609,525,000		582,736,000	0		332,665,000	249,525,000		249,525,000								360,000,000		0
Unamortized portion of debt issue costs				4,185,000			3,627,000			10,655,000							7,547,000
Line of Credit Facility, Expiration Date												10/18/2012			6/10/2016
Conditions of determining interest rate															Interest is determined, at our election, by reference to (a) the British Bankers Association LIBOR rate, or LIBOR, plus an applicable rate between 2.0% and 3.25% per annum or (b) the highest of (1) the federal funds rate plus 0.50%, (2) the prime rate and (3) LIBOR plus 1.0%, plus, in each case, an applicable rate between 1.0% and 2.25% per annum
Excess of interest rate over LIBOR rate																						3.25%	2.00%
Applicable interest rate																						2.25%	1.00%
Quarterly commitment fee applicable rate																						0.50%	0.375%
Quarterly commitment fee															Determined by reference to an applicable rate between 0.375% and 0.5% per annum
Interest rate stated percentage				8.125%			7.75%										7.125%
Public offering closed in aggregate principle amount																		360,000,000
Loss on tender and redemption on the notes				18,233,000
Financing fees paid															$ 7,833,000
Long Term Debt (Textual) [Abstract]
Excess of interest rate over federal funds rate	5.00%
Excess of interest rate over LIBOR rate Condition two	1.00%
Rate of quarterly commitment fee on the unused portion of the revolving credit facility	0.375%	0.25%
Effective average interest rate on borrowings under the credit facility	6.48%	7.74%