Summarized Quarterly Financial Information - Unaudited (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																9 Months Ended	12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Sep. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 38,186		$ 33,951		$ 33,413		$ 36,041		$ 38,093		$ 39,029		$ 41,975		$ 41,603			$ 141,591	$ 160,700	$ 179,263
Operating Income (Loss)	(24,027)	[1]	(35,919)	[1]	(52,183)	[1]	(18,314)	[1]	2,899	[2]	4,749	[2]	(2,088)	[2]	3,178	[2]		(130,443)	8,738	48,756
Net income (loss) available to common stockholders	(25,451)	[1]	(38,639)	[1]	(54,520)	[1]	(18,608)	[1]	2,830	[2]	3,727	[2]	(3,045)	[2]	1,897	[2]	5,409	(137,218)	5,409	41,987
Earnings per share, Basic (in usd per share)	$ (0.41)		$ (0.62)		$ (0.87)		$ (0.30)		$ 0.04		$ 0.06		$ (0.05)		$ 0.03			$ (2.20)	$ 0.08	$ 0.67
Earnings per share, Diluted (in usd per share)	$ (0.41)		$ (0.62)		$ (0.87)		$ (0.30)		$ 0.04		$ 0.06		$ (0.05)		$ 0.03			$ (2.20)	$ 0.08	$ 0.66
Increase (Decrease) in ceiling test write-downs	$ 28,100		$ 35,400		$ 53,500		$ 20,100						$ 13,000		$ 5,900			$ 137,100	$ 18,907	$ 0

[1] Loss from operations and net loss available to common stockholders reported during the three months ended March 31, June 30, September 30 and December 31, 2012 included ceiling test write-downs of $20.1 million, $53.5 million, $35.4 million and $28.1 million, respectively.

[2] Income (loss) from operations and net income (loss) available to common stockholders reported during the three months ended March 31 and June 30, 2011 included ceiling test write-downs of $5.9 million and $13.0 million, respectively.