Quarterly Results of Operations (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
Effect of Fourth Quarter Events [Line Items]                      
Revenue $ 147,259 $ 162,654 $ 171,522 $ 157,270 $ 162,225 $ 182,768 $ 193,470 $ 212,986 $ 638,705 $ 751,449 $ 943,405
Operating income (loss) (10,826) 10,581 22,414 (7,858) (9,870) 19,524 19,133 39,676 14,311 68,463 191,627
Net income (loss) (5,248) 6,291 12,955 (4,330) (5,109) 14,211 12,114 24,667 9,668 45,883 121,146
Earnings per common share:                      
Basic (in USD per share) $ (0.12) $ 0.14 $ 0.29 $ (0.10) $ (0.10) $ 0.26 $ 0.22 $ 0.46 $ 0.21 $ 0.85 $ 2.29
Diluted (in USD per share) $ (0.12) $ 0.14 $ 0.28 $ (0.10) $ (0.10) $ 0.25 $ 0.22 $ 0.45 $ 0.21 $ 0.83 $ 2.17
Year-End Adjustment [Member]                      
Effect of Fourth Quarter Events [Line Items]                      
Revenue (1,700)               (1,000)    
Operating income (loss) (1,700)               (700)    
Net income (loss) $ (1,000) $ 200             $ (200)