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Impact of change in accounting policies Statement of operations (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2014
Revenues [Abstract]            
Revenue   $ 591,781   $ 1,169,847    
Reimbursable expenses   160,758   306,870    
Total Revenue $ 641,610 431,023 $ 1,261,735 862,977    
Costs and expenses [Abstract]            
Direct costs    
Direct Costs, Reimbursable Expenses 169,313 0 328,066 0    
Direct Costs, Other 279,913 250,044 551,752 500,503    
Selling, general and administrative expense 80,936 80,833 161,851 162,222    
Depreciation and amortization 17,046 14,395 33,944 28,843    
Restructuring charges 12,490 7,753 12,490 7,753 $ 7,800 $ 8,800
Total costs and expenses 559,698 353,025 1,088,103 699,321    
Income from operations 81,912 77,998 173,632 163,656    
Interest income 1,017 579 1,840 1,134    
Interest expense (3,302) (3,179) (7,097) (6,358)    
Income before provision for income taxes 79,627 75,398 168,375 158,432    
Provision for income taxes (7,759) (10,581) (18,409) (22,206)    
Net income 71,868 $ 64,817 149,966 $ 136,226    
Difference between Revenue Guidance in Effect before and after Topic 606            
Costs and expenses [Abstract]            
Net income     2,334      
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09            
Revenues [Abstract]            
Revenue 1,614   2,845      
Reimbursable expenses 169,313   (328,066)      
Total Revenue (167,699)   (325,221)      
Costs and expenses [Abstract]            
Direct costs        
Direct Costs, Reimbursable Expenses (169,313)   (328,066)      
Direct Costs, Other 0   0      
Selling, general and administrative expense 62   222      
Depreciation and amortization 0   0      
Restructuring charges 0   0      
Total costs and expenses (169,251)   (327,844)      
Income from operations 1,552   2,623      
Interest income 0   0      
Interest expense 0   0      
Income before provision for income taxes 1,552   2,623      
Provision for income taxes (155)   289      
Net income 1,397   2,334      
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09            
Revenues [Abstract]            
Revenue 643,224   1,264,580      
Reimbursable expenses 169,313   328,066      
Total Revenue 473,911   936,514      
Costs and expenses [Abstract]            
Direct costs          
Direct Costs, Reimbursable Expenses 0   0      
Direct Costs, Other 279,913   551,752      
Selling, general and administrative expense 80,998   162,073      
Depreciation and amortization 17,046   33,944      
Restructuring charges 12,490   12,490      
Total costs and expenses 390,447   760,259      
Income from operations 83,464   176,255      
Interest income 1,017   1,840      
Interest expense (3,302)   (7,097)      
Income before provision for income taxes 81,179   170,998      
Provision for income taxes (7,914)   (18,698)      
Net income $ 73,265   $ 152,300