Summary of Significant Accounting Policies - New Accounting Pronouncement (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2019	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in revenues	$ 2,951	$ 2,846	$ 8,996	$ 8,324
Increase in management fees	474	419	1,411	1,198
Increase in trading services revenue	288	259	907	823
Increase in expenses	2,079	2,021	6,494	6,138
Increase in transaction processing services	236	215	724	619
Increase in other expenses	$ 172	$ 144	579	$ 427
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			205
Increase in management fees			140
Increase in trading services revenue			47
Increase in other revenues			18
Increase in expenses			205
Increase in transaction processing services			48
Increase in other expenses			128
Increase in all other expenses			$ 29
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
Forecast | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Future minimum operating lease payments due					$ (1,000)