Selected Quarterly Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Data (Unaudited)
Revenues	$ 146,518us-gaap_Revenues	$ 134,121us-gaap_Revenues	$ 140,310us-gaap_Revenues	$ 132,727us-gaap_Revenues	$ 152,179us-gaap_Revenues	$ 134,973us-gaap_Revenues	$ 140,105us-gaap_Revenues	$ 132,352us-gaap_Revenues	$ 553,676us-gaap_Revenues	$ 559,609us-gaap_Revenues	$ 581,091us-gaap_Revenues
Operating income (loss)	14,656us-gaap_OperatingIncomeLoss	10,540us-gaap_OperatingIncomeLoss	10,987us-gaap_OperatingIncomeLoss	4,579us-gaap_OperatingIncomeLoss	12,484us-gaap_OperatingIncomeLoss	8,360us-gaap_OperatingIncomeLoss	12,413us-gaap_OperatingIncomeLoss	9,404us-gaap_OperatingIncomeLoss	40,762us-gaap_OperatingIncomeLoss	42,661us-gaap_OperatingIncomeLoss	62,669us-gaap_OperatingIncomeLoss
Income (loss) from continuing operations	10,089	6,420	5,637	1,845	6,734	4,451	6,936	6,320	23,991	24,441	35,396
Income (loss) from discontinued operations					(168)	(12,624)	1,373	348		(11,071)	(118,749)
Net income (loss)	$ 10,089	$ 6,420	$ 5,637	$ 1,845	$ 6,566	$ (8,173)	$ 8,309	$ 6,668	$ 23,991	$ 13,370	$ (83,353)
Basic earnings (loss) per common share
Continuing operations (in dollars per share)	$ 0.16	$ 0.10	$ 0.09	$ 0.03	$ 0.11	$ 0.07	$ 0.11	$ 0.10	$ 0.38	$ 0.39	$ 0.56
Discontinued operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ (0.20)	$ 0.02	$ 0.01	$ 0.00	$ (0.18)	$ (1.89)
Basic earnings (loss) per common share (in dollars per share)	$ 0.16us-gaap_EarningsPerShareBasic	$ 0.10us-gaap_EarningsPerShareBasic	$ 0.09us-gaap_EarningsPerShareBasic	$ 0.03us-gaap_EarningsPerShareBasic	$ 0.10us-gaap_EarningsPerShareBasic	$ (0.13)us-gaap_EarningsPerShareBasic	$ 0.13us-gaap_EarningsPerShareBasic	$ 0.11us-gaap_EarningsPerShareBasic	$ 0.38us-gaap_EarningsPerShareBasic	$ 0.21us-gaap_EarningsPerShareBasic	$ (1.33)us-gaap_EarningsPerShareBasic
Diluted earnings (loss) per common share
Continuing operations (in dollars per share)	$ 0.16	$ 0.10	$ 0.09	$ 0.03	$ 0.11	$ 0.07	$ 0.11	$ 0.10	$ 0.38	$ 0.39	$ 0.56
Discontinued operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ (0.20)	$ 0.02	$ 0.01	$ 0.00	$ (0.18)	$ (1.88)
Diluted earnings (loss) per common share (in dollars per share)	$ 0.16us-gaap_EarningsPerShareDiluted	$ 0.10us-gaap_EarningsPerShareDiluted	$ 0.09us-gaap_EarningsPerShareDiluted	$ 0.03us-gaap_EarningsPerShareDiluted	$ 0.10us-gaap_EarningsPerShareDiluted	$ (0.13)us-gaap_EarningsPerShareDiluted	$ 0.13us-gaap_EarningsPerShareDiluted	$ 0.11us-gaap_EarningsPerShareDiluted	$ 0.38us-gaap_EarningsPerShareDiluted	$ 0.21us-gaap_EarningsPerShareDiluted	$ (1.32)us-gaap_EarningsPerShareDiluted

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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