Selected Quarterly Data (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
Selected Quarterly Data (Unaudited)
Revenues	$ 204,835	$ 187,203	$ 189,629	$ 186,042	$ 215,113	$ 202,967	$ 202,838	$ 190,718	$ 767,709	$ 811,636	$ 820,720
Operating income (loss)	22,142	17,345	(143,403)	13,976	24,622	21,178	17,234	15,064	(89,940)	78,098	93,246
Income (loss) from continuing operations	14,343	9,756	(104,638)	7,435	15,310	12,263	9,954	8,350	(73,104)	45,877	55,194
Loss from discontinued operations	(3,647)	(893)	(5,067)	(642)	(582)	(135)	(529)	(433)	(10,249)	(1,679)	(1,590)
Net income (loss)	$ 10,696	$ 8,863	$ (109,705)	$ 6,793	$ 14,728	$ 12,128	$ 9,425	$ 7,917	$ (83,353)	$ 44,198	$ 53,604
Basic earnings per share:
Continuing operations (in dollars per share)	$ 0.23	$ 0.15	$ (1.66)	$ 0.12	$ 0.24	$ 0.20	$ 0.16	$ 0.13	$ (1.16)	$ 0.73	$ 0.87
Discontinued operations (in dollars per share)	$ (0.06)	$ (0.01)	$ (0.08)	$ (0.01)	$ (0.01)	$ (0.01)	$ (0.01)	$ (0.01)	$ (0.17)	$ (0.03)	$ (0.03)
Basic earnings (loss) per common share (in dollars per share)	$ 0.17	$ 0.14	$ (1.74)	$ 0.11	$ 0.23	$ 0.19	$ 0.15	$ 0.12	$ (1.33)	$ 0.70	$ 0.84
Diluted earnings per share
Continuing operations (in dollars per share)	$ 0.23	$ 0.15	$ (1.66)	$ 0.12	$ 0.24	$ 0.19	$ 0.16	$ 0.13	$ (1.16)	$ 0.72	$ 0.86
Discontinued operations (in dollars per share)	$ (0.06)	$ (0.01)	$ (0.08)	$ (0.01)	$ (0.01)	$ 0.00	$ (0.01)	$ (0.01)	$ (0.17)	$ (0.02)	$ (0.02)
Diluted earnings (loss) per common share (in dollars per share)	$ 0.17	$ 0.14	$ (1.74)	$ 0.11	$ 0.23	$ 0.19	$ 0.15	$ 0.12	$ (1.33)	$ 0.70	$ 0.84