Summary of Significant Accounting Policies - New Accounting Pronouncement (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in revenues	$ 3,019	$ 2,668
Increase in management fees	472	382
Increase in trading services revenue	304	275
Increase in expenses	2,256	2,086
Increase in transaction processing services	242	197
Increase in other expenses	201	$ 151
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	65
Increase in management fees	45
Increase in trading services revenue	15
Increase in other revenues	5
Increase in expenses	65
Increase in transaction processing services	15
Increase in other expenses	45
Increase in all other expenses	$ 5
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