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Investments (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended
Mar. 31, 2015
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Jun. 30, 2016
Investments, Debt and Securities [Line Items]          
Trading securities   $ 203   $ 228  
Cumulative unrealized gains and losses   0 $ 0    
Net income from unconsolidated investees   $ 4 $ 2    
Borsa Istanbul Cost Method Investment          
Investments, Debt and Securities [Line Items]          
Ownership interest, cost method investments   5.00%   5.00%  
Cost method investment, original amount   $ 75      
LCH          
Investments, Debt and Securities [Line Items]          
Ownership interest, cost method investments   5.00%   5.00%  
OCC          
Investments, Debt and Securities [Line Items]          
Committed equity contribution $ 150        
Capital contribution $ 30        
Annual dividend from shareholders percentage 50.00%        
OCC | ISE          
Investments, Debt and Securities [Line Items]          
Ownership interest, equity method investments   40.00%     20.00%
Foreign Government Debt Securities          
Investments, Debt and Securities [Line Items]          
Trading securities   $ 154   $ 172