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Acquisitions and Dispositions - ETG - Additional Information (Details) - USD ($)
3 Months Ended 7 Months Ended 12 Months Ended
May 15, 2014
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Business Acquisition [Line Items]                          
Revenue included in consolidated statements of operations   $ 309,879,000 $ 275,659,000 $ 261,994,000 $ 255,658,000 $ 252,262,000 $ 223,031,000 $ 219,349,000 $ 201,735,000   $ 1,103,190,000 $ 896,377,000 $ 631,311,000
Operating income (loss)   $ 92,475,000 $ 72,897,000 $ 64,007,000 $ 57,929,000 $ 67,346,000 $ 48,315,000 $ 52,542,000 $ 37,904,000   $ 287,308,000 $ 206,107,000 173,237,000
ETG [Member]                          
Business Acquisition [Line Items]                          
Acquisition date May 15, 2014                        
Purchase price of entities acquired $ 7,200,000                        
Acquisition related costs                         $ 0
Revenue included in consolidated statements of operations                   $ 3,000,000      
Operating income (loss)                   $ (200,000)