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Commitments and Contingencies - Lease and Other Commitments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Contingencies - Legal Matters      
Rent expense $ 11,800 $ 12,700 $ 12,400
Maximum separation payments payable for early termination of employment arrangements without cause 6,000    
Obligation to purchase market data, maintenance and other services 63,600    
Minimum future rental commitments under non-cancelable operating leases      
2018 13,514    
2019 13,542    
2020 12,161    
2021 11,559    
2022 10,402    
2023 and thereafter 60,301    
Total $ 121,479