Long-Term Debt (Details Textual) (USD $)	3 Months Ended		3 Months Ended			3 Months Ended							3 Months Ended						3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Facility Agreement [Member]	Dec. 31, 2011 Facility Agreement [Member]	Dec. 17, 2009 Facility Agreement [Member]	Mar. 31, 2012 Restructured Facility Agreement [Member]	Mar. 31, 2012 Secured Reducing Revolving Loan Facility [Member]	Dec. 31, 2011 Secured Reducing Revolving Loan Facility [Member]	Mar. 31, 2012 Secured Reducing Revolving Loan Facility [Member] Maximum [Member]	Mar. 31, 2012 Secured Reducing Revolving Loan Facility [Member] Minimum [Member]	Mar. 31, 2012 Senior Notes Due 2014 [Member]	Dec. 31, 2011 Senior Notes Due 2014 [Member]	Mar. 31, 2012 Senior Notes Due 2014 [Member] 7.75% Senior notes [Member]	Jul. 15, 2012 Senior Notes Due 2014 [Member] 7.75% Senior notes [Member]	Apr. 02, 2012 Senior Notes Due 2014 [Member] 7.75% Senior notes [Member]	Jul. 21, 2004 Senior Notes Due 2014 [Member] 7.75% Senior notes [Member]	Mar. 31, 2012 Senior Notes Due 2022 [Member]	Mar. 12, 2012 Senior Notes Due 2022 [Member] 6.375% Senior notes [Member]	Mar. 31, 2012 Senior Notes Due 2022 [Member] 6.375% Senior notes [Member] Scenario, Forecast [Member]	Mar. 15, 2017 Senior Notes Due 2022 [Member] 6.375% Senior notes [Member] Scenario, Forecast [Member]	Mar. 15, 2020 Senior Notes Due 2022 [Member] 6.375% Senior notes [Member] Scenario, Forecast [Member]	Mar. 15, 2019 Senior Notes Due 2022 [Member] 6.375% Senior notes [Member] Scenario, Forecast [Member]	Mar. 15, 2018 Senior Notes Due 2022 [Member] 6.375% Senior notes [Member] Scenario, Forecast [Member]	Mar. 15, 2017 Senior Notes Due 2022 [Member] 6.375% Senior notes [Member] Scenario, Forecast [Member]
Long-Term Debt (Textual) [Abstract]
Aggregate principal amount																$ 160,000,000		$ 300,000,000
Debt Instrument, Interest Rate, Stated Percentage																7.75%		6.375%
Latest call date of senior notes													Jul. 15, 2012
Redemption price of senior notes													101.292%	100.00%
Redemption price as percentage of principal amount premium and interest																				100.00%	100.00%	101.063%	102.125%	103.188%
Fair value of senior notes on the basis of quoted market prices													80,300,000				301,500,000
Maximum borrowing capacity					200,000,000		175,000,000
Quarterly installment for repaying borrowed amount under facility agreement						8,300,000	15,000,000
Final installment for repaying borrowed amount under facility agreement						108,300,000
Margin over LIBOR rate			2.50%						0.90%	0.70%										0.50%				0.50%
Basis on make-whole premium																				U.S. treasuries plus 50 basis points
Debt issuance cost																			7,500,000
Current borrowing capacity							153,000,000
Final amount of reduction in credit facility							130,000,000
Amount drawn under facility							7,000,000
Maturity date	Jul. 01, 2014
Principal amount of old debt under tender offer													1,000
Tender offer price for principal amount of old notes													1,015.50
Consent payment included in consideration under tender offer													15.00
Redemption date of outstanding old notes after tender offer													Apr. 02, 2012
Remaining amount of debt	426,770,000	307,667,000	40,000,000	141,667,000			7,028,000	6,000,000			79,742,000	160,000,000	79,700,000				300,000,000
Net amount outstanding under facility agreement	140,000,000
Repayment related to outstanding under the Facility Agreement	100,000,000
Payment of indebtedness related to the facility agreement			100,000,000
Redemption price of principal amount															$ 1,012.92