Supplemental Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 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		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 101,610		$ 107,763		$ 104,610		$ 109,069		$ 106,648		$ 101,064		$ 109,543		$ 109,173		$ 423,052		$ 426,428
Operating income	10,588		21,573		20,837		26,795		9,633		32,878		45,258		44,036		79,793		131,805		77,944
Net income (loss)	$ 1,678		$ (7,176)		$ 32,822		$ 7,779		$ (15,493)		$ 74,523		$ 42,668		$ (7,875)		$ 35,103		$ 93,823
Net income (loss) per common share:
Basic (in dollars per share)	$ 0.14	[1]	$ (0.59)	[1]	$ 2.70	[1]	$ 0.64	[1]	$ (1.27)	[1]	$ 6.13	[1]	$ 3.51	[1]	$ (0.65)	[1]	$ 2.89	[1]	$ 7.72	[1]	$ 3.04
Diluted (in dollars per share)	$ 0.14	[1]	$ (0.59)	[1]	$ 2.70	[1]	$ 0.64	[1]	$ (1.27)	[1]	$ 6.13	[1]	$ 3.51	[1]	$ (0.65)	[1]	$ 2.89	[1]	$ 7.71	[1]	$ 3.04
Weighted average common shares outstanding:
Basic shares	12,164		12,164		12,164		12,164		12,163		12,163		12,162		12,156		12,164		12,161		12,148
Diluted shares	12,164		12,164		12,164		12,164		12,163		12,163		12,163		12,156		12,164		12,162		12,148

[1] The sum of the individual quarterly net income (loss) per share amounts may not agree to the total for the year since each period’s computation is based on the weighted average number of common shares outstanding during each period.