Summary of Significant Accounting Policies New and Recently Adopted Accounting Pronouncements (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended |
|---|---|---|
Jun. 30, 2016 |
Jun. 30, 2016 |
|
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||
| Unrealized Gain (Loss) on Securities | $ 75 | $ 128 |
| Cetip, S.A. [Member] | ||
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||
| Available-for-sale Securities, Gross Unrealized Gain (Loss) | $ 103 |