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Significant Accounting Policies - Property and Equipment (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Property and equipment      
Depreciation expenses $ 13.3 $ 19.5 $ 21.8
Computers and equipment | Minimum      
Property and equipment      
Estimated useful lives of assets 3 years    
Computers and equipment | Maximum      
Property and equipment      
Estimated useful lives of assets 4 years    
Furniture and fixtures      
Property and equipment      
Estimated useful lives of assets 5 years