Description of Business and Significant Accounting Policies - Impact of the Adoption of Topic 606 on the Condensed Consolidated Income Statements (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Jan. 01, 2018
Dec. 31, 2017
Revenue        
Professional service revenues $ 129 $ 120    
Total revenues 861 808    
Expense        
Commissions expense 4      
Total expense 790 758    
Income before provision for income taxes 67 45    
Income tax expense 13 16    
Net income $ 54 $ 29    
Basic EPS (in dollars per share) $ 0.77 $ 0.42    
Diluted EPS (in dollars per share) $ 0.75 $ 0.41    
Assets        
Unbilled revenue (net of advance collections) $ 286     $ 297
Prepaid expenses and other current assets 22     15
Other assets 18     16
Accounts payable and other current liabilities 46     59
Liabilities        
Other liabilities 10     14
Equity        
Retained earnings (332)     $ (377)
Balance using Previous Standard        
Revenue        
Professional service revenues 126      
Total revenues 858      
Expense        
Commissions expense 13      
Total expense 799      
Income before provision for income taxes 56      
Income tax expense 11      
Net income $ 45      
Basic EPS (in dollars per share) $ 0.64      
Diluted EPS (in dollars per share) $ 0.62      
Assets        
Unbilled revenue (net of advance collections) $ 291      
Prepaid expenses and other current assets 14      
Other assets 14      
Liabilities        
Accounts payable and other current liabilities 47      
Other liabilities 12      
Equity        
Retained earnings (342)      
ASU 2014-09 | Increase (Decrease)        
Revenue        
Professional service revenues 3      
Total revenues 3      
Expense        
Commissions expense (9)      
Total expense (9)      
Income before provision for income taxes 11      
Income tax expense 2      
Net income $ 9      
Basic EPS (in dollars per share) $ 0.13      
Diluted EPS (in dollars per share) $ 0.13      
Assets        
Unbilled revenue (net of advance collections) $ (5)      
Prepaid expenses and other current assets 8      
Other assets 4      
Liabilities        
Accounts payable and other current liabilities (1)      
Other liabilities (2)      
Equity        
Retained earnings $ 10   $ 1