v3.19.3.a.u2
Subsequent Events - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Feb. 13, 2020
Jan. 29, 2020
Jan. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Subsequent Event [Line Items]            
Debt instrument, aggregate principal amount       $ 4,986    
Dividend declared date       Jan. 29, 2020    
Operating expenses       $ 8,988 $ 8,741 $ 8,346
Tax benefit       $ (1,261) $ (1,076) $ (270)
Subsequent Event [Member]            
Subsequent Event [Line Items]            
Quarterly dividend payable, per share   $ 3.63        
Dividend payable date   Mar. 23, 2020        
Dividend payable, record date   Mar. 05, 2020        
Subsequent Event [Member] | BlackRock Foundation [Member]            
Subsequent Event [Line Items]            
Operating expenses $ 589          
Operating expenses expected to be offset by noncash 125          
Tax benefit $ 241          
2.40% Notes due 2030 [Member] | Subsequent Event [Member]            
Subsequent Event [Line Items]            
Debt instrument, aggregate principal amount     $ 1,000      
Debt instrument, interest rate     2.40%      
Debt instrument, maturity date     Apr. 30, 2030      
Debt instrument, approximate annual interest expense     $ 24      
Debt instrument, payment terms     Interest of approximately $24 million per year will be payable semi-annually on April 30 and October 30 of each year, commencing April 30, 2020      
Debt instrument, redemption period, end date     Jan. 30, 2030      
Debt Instrument, redemption price, percentage of principal amount redeemed     100.00%