Quarterly Supplemental Financial Information (unaudited) (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 Data [Abstract]
Revenues	$ 213,317		$ 206,581		$ 199,646		$ 193,699		$ 191,554		$ 187,574		$ 181,804		$ 144,076		$ 813,243	$ 705,008		$ 591,072
Income (loss) before income taxes	(7,999)		10,020		19,177		24,876		21,816		(10,474)		(30,996)		6,939		46,074	(12,715)		58,383
Net income (loss)	$ (2,418)		$ 8,730		$ 14,716		$ 19,184		$ 16,891		$ (4,776)		$ (17,914)		$ 5,810		$ 40,212	$ 11		$ 47,513
Basic earnings (loss) per share	$ (0.10)	[1]	$ 0.34	[1]	$ 0.58	[1]	$ 0.75	[1]	$ 0.66	[1]	$ (0.19)	[1]	$ (0.69)	[1]	$ 0.22	[1]	$ 1.58	$ 0.00	[1]	$ 1.81
Diluted earnings (loss) per share	$ (0.10)	[1]	$ 0.34	[1]	$ 0.58	[1]	$ 0.75	[1]	$ 0.66	[1]	$ (0.19)	[1]	$ (0.69)	[1]	$ 0.22	[1]	$ 1.58	$ 0.00	[1]	$ 1.80

[1] The sum of the quarterly reported amounts may not equal the full year, as each is computed independently.