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Quarterly financial data (unaudited) (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Quarterly financial data (unaudited) [Abstract]                      
Revenue $ 193,809 $ 149,434 $ 100,772 $ 116,858 $ 151,663 $ 142,914 $ 91,241 $ 102,964 $ 560,873 $ 488,782 $ 489,331
Income (loss) from operations 53,077 47,769 8,529 14,910 50,929 35,478 (2,042) 21,157      
Net income from continuing operations $ 33,942 $ 33,551 $ 4,616 $ 10,485 32,289 20,388 (2,042) 11,889 82,594 62,524 91,416
Net loss from discontinued operations         5,103 [1] 952 [1] (8,905) [1] (7,898) [1] $ 0 $ (10,748) $ (28,546)
Net income (loss)         $ 37,392 $ 21,340 $ (10,947) $ 3,991      
Net income (loss) per share from continuing operations-basic (in dollars per share) $ 0.77 $ 0.81 $ 0.11 $ 0.26 $ 0.80 $ 0.50 $ (0.05) $ 0.30 $ 1.98 $ 1.56 $ 2.37
Net loss per share from discontinued operations-basic (in dollars per share)         0.13 0.02 (0.22) (0.20) 0 (0.27) (0.74)
Net income per share-basic (in dollars per share)         0.93 0.52 (0.27) 0.10 1.98 1.29 1.63
Net income per share from continuing operations-diluted (in dollars per share) $ 0.67 $ 0.68 $ 0.11 $ 0.23 0.67 0.43 (0.05) 0.26 1.71 1.35 2.02
Net income (loss) per share from discontinued operations-diluted (in dollars per share)         0.10 0.02 (0.22) (0.16) 0 (0.22) (0.61)
Net income per share-diluted (in dollars per share)         $ 0.77 $ 0.45 $ (0.27) $ 0.10 $ 1.71 [2] $ 1.13 [2] $ 1.41 [2]
[1] Reflects a change in estimate attributed to higher pretax income within continuing operations. According to the ordering rules of intraperiod tax allocation, the residual amount of change after determining the effective rate for continuing operations is allocated to discontinued operations.
[2] See "Earnings per share" footnote for details on calculation.