Quarterly Financial Data (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Quarterly Financial Data (unaudited)
Revenue	$ 112,561	$ 125,809	$ 217,268	$ 242,416
Operating income	12,558	20,366	20,355	35,634
Net income and comprehensive income	$ 6,038	$ 10,420	$ 9,391	$ 17,396
Income per common share - Basic (in dollars per share)	$ 0.07	$ 0.12	$ 0.11	$ 0.20
Income per common share - Diluted (in dollars per share)	$ 0.07	$ 0.12	$ 0.11	$ 0.20