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