Quarterly report pursuant to Section 13 or 15(d)

Property and Equipment - Additional Information (Details)

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Property and Equipment - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Property Plant And Equipment [Abstract]        
Depreciation expense on property and equipment $ 54,075 $ 53,344 $ 105,879 $ 107,527
Disposal of property and equipment     $ 8,239 $ 13,518