Summary of Significant Accounting Policies - New Accounting Pronouncement (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in revenues	$ 3,026	$ 2,810	$ 6,045	$ 5,478
Increase in management fees	465	397	937	779
Increase in trading services revenue	315	289	619	564
Increase in expenses	2,159	2,031	4,415	4,117
Increase in transaction processing services	246	207	488	404
Increase in other expenses	$ 206	$ 132	407	$ 283
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in revenues			135
Increase in management fees			90
Increase in trading services revenue			35
Increase in other revenues			10
Increase in expenses			65
Increase in transaction processing services			30
Increase in other expenses			90
Increase in all other expenses			$ 15
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Money market funds reclassified					$ 397
Available-for-sale equity securities reclassified					$ 46