Long-Term Debt (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Schedule of Capitalization, Long-term Debt [Line Items]
Borrowing capacity of new revolving credit agreement	$ 450,000,000
Credit agreement, maturity date	April 2016
Debt Instrument, Maturity Date	May 28, 2020
Fixed rate senior secured notes	75,000	75,000
Senior secured notes, interest rate	6.04%
Book Value of Collateralized By Certain Assets	76,254,000
Long-term Debt, by Maturity [Abstract]
Repayments of Principal in Next Twelve Months	10,800,000
Repayments of Principal after Year Five	45,128,000
Long-term Debt, Maturing in Years Two and Three [Abstract]
Repayments of Principal in Year Two	13,471,000
Repayments of Principal in Year Three	14,609,000
Long-term Debt, Maturing in Years Four and Five [Abstract]
Repayments of Principal in Year Four	11,936,000
Repayments of Principal in Year Five	$ 362,819,000
Maximum [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt instruments interest rate base rate plus	1.75%
Debt instruments interest rate Libor plus	2.75%
Unused capacity commitment fee, percentage	0.50%
Minimum [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt instruments interest rate base rate plus	0.75%
Debt instruments interest rate Libor plus	1.75%
Unused capacity commitment fee, percentage	0.20%