v3.19.3.a.u2
Significant Accounting Policies - Additional Information (Detail)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2019
USD ($)
Segment
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Significant Accounting Policies [Line Items]          
Operating lease right-of-use asset     $ 669    
Operating lease liability     $ 776    
Cash and cash equivalents maturities     three months or less    
Fair value of loaned securities     $ 14,400 $ 18,900  
Fair value of collateral held for loan securities     15,500 20,700  
Discrete income tax benefit for vested stock awards $ 22 $ 56 $ 28 64  
Number of business segments | Segment     1    
Restricted Stock Units (RSUs) [Member]          
Significant Accounting Policies [Line Items]          
Discrete income tax benefit for vested stock awards     $ 23 $ 64 $ 151
Minimum [Member]          
Significant Accounting Policies [Line Items]          
VIE, economic interest percentage     10.00%    
Collateral cash and securities received in exchange of value of securities lent in order to reduce counterparty risk     102.00%    
Maximum [Member]          
Significant Accounting Policies [Line Items]          
Collateral cash and securities received in exchange of value of securities lent in order to reduce counterparty risk     112.00%    
Maximum [Member] | Software [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life     3 years    
Accounting Standards Update 2016-02 [Member]          
Significant Accounting Policies [Line Items]          
Operating lease right-of-use asset     $ 700    
Operating lease liability     $ 700