Summary of Significant Accounting Policies New and Recently Adopted Accounting Pronouncements (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2016 |
Sep. 30, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
|
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||||
| Unrealized Gain (Loss) on Securities | $ (12) | $ 117 | ||
| Cetip, S.A. [Member] | ||||
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||||
| Available-for-sale Securities, Gross Unrealized Gain (Loss) | $ 91 | |||
| Transaction Based Expenses [Domain] | ||||
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||||
| Prior Period Reclassification Adjustment | $ 36 | $ 83 | ||