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Summary of significant accounting policies (Details)
$ in Thousands
12 Months Ended
May 19, 2016
shares
Dec. 31, 2016
USD ($)
Plan
shares
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Unbilled accounts receivable   $ 138,478 $ 113,906  
Capitalized interest [Abstract]        
Interest costs incurred   8,300 7,800 $ 7,500
Interest costs capitalized   $ 2,200 $ 2,900 $ 2,500
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Increase in number of shares of common stock available for issuance (in shares) | shares 3,800,000      
Buildings [Member] | Minimum [Member]        
Property, Plant and Equipment [Line Items]        
Estimated useful lives   31 years    
Buildings [Member] | Maximum [Member]        
Property, Plant and Equipment [Line Items]        
Estimated useful lives   39 years    
Building Improvements [Member] | Minimum [Member]        
Property, Plant and Equipment [Line Items]        
Estimated useful lives   10 years    
Building Improvements [Member] | Maximum [Member]        
Property, Plant and Equipment [Line Items]        
Estimated useful lives   39 years    
Furniture and Equipment [Member] | Minimum [Member]        
Property, Plant and Equipment [Line Items]        
Estimated useful lives   3 years    
Furniture and Equipment [Member] | Maximum [Member]        
Property, Plant and Equipment [Line Items]        
Estimated useful lives   15 years    
Software [Member]        
Property, Plant and Equipment [Line Items]        
Estimated useful lives   3-7 years or product life    
Leasehold Improvements [Member]        
Property, Plant and Equipment [Line Items]        
Estimated useful lives   Lesser of the asset life or lease term    
2006 Plan [Member] | Stock Options [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Common stock authorized for issuance under the plan (in shares) | shares   18,900,000    
Common stock available for future awards (in shares) | shares   6,100,000    
Contractual life of awards   10 years    
Exercise period of options, maximum   10 years    
Assumptions used in valuing the stock options granted [Abstract]        
Expected dividend yield   0.00% 0.00% 0.00%
Expected average life of options   4 years 3 months 18 days 4 years 3 months 18 days 4 years 6 months
2006 Plan [Member] | Stock Options [Member] | Minimum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercise price of option as percentage of fair market value at grant date, minimum   100.00%    
Assumptions used in valuing the stock options granted [Abstract]        
Expected volatility   31.00% 34.00% 35.00%
Risk-free interest rate   0.93% 1.27% 1.14%
2006 Plan [Member] | Stock Options [Member] | Maximum [Member]        
Assumptions used in valuing the stock options granted [Abstract]        
Expected volatility   33.00% 35.00% 38.00%
Risk-free interest rate   1.22% 1.61% 1.65%
Emergent Plans [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of stock based employee compensation plans | Plan   2    
Revenues [Member] | U.S. Government [Member]        
Concentration Risk [Line Items]        
Concentration risk percentage   83.00% 86.00% 83.00%
Accounts Receivable [Member] | U.S. Government [Member]        
Concentration Risk [Line Items]        
Concentration risk percentage   83.00% 83.00%  
Unbilled [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Unbilled accounts receivable   $ 48,039 $ 18,171