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Investments - Investments in Funds that are Calculated Using Net Asset Value Per Share (Detail) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Cash Instruments Assets [Member]    
Fair Value, Investments, Entities That Are Calculated Using Net Asset Value Per Share [Line Items]    
Fair Value of Investments $ 4,206 $ 3,936
Cash Instruments Liabilities [Member]    
Fair Value, Investments, Entities That Are Calculated Using Net Asset Value Per Share [Line Items]    
Unfunded Commitments 1,781 2,111
Private Equity Funds [Member] | Cash Instruments Assets [Member]    
Fair Value, Investments, Entities That Are Calculated Using Net Asset Value Per Share [Line Items]    
Fair Value of Investments 2,767 2,683
Private Equity Funds [Member] | Cash Instruments Liabilities [Member]    
Fair Value, Investments, Entities That Are Calculated Using Net Asset Value Per Share [Line Items]    
Unfunded Commitments 765 809
Credit Funds [Member] | Cash Instruments Assets [Member]    
Fair Value, Investments, Entities That Are Calculated Using Net Asset Value Per Share [Line Items]    
Fair Value of Investments 983 548
Credit Funds [Member] | Cash Instruments Liabilities [Member]    
Fair Value, Investments, Entities That Are Calculated Using Net Asset Value Per Share [Line Items]    
Unfunded Commitments 820 1,099
Hedge Funds [Member] | Cash Instruments Assets [Member]    
Fair Value, Investments, Entities That Are Calculated Using Net Asset Value Per Share [Line Items]    
Fair Value of Investments 125 161
Real Estate Funds [Member] | Cash Instruments Assets [Member]    
Fair Value, Investments, Entities That Are Calculated Using Net Asset Value Per Share [Line Items]    
Fair Value of Investments 331 544
Real Estate Funds [Member] | Cash Instruments Liabilities [Member]    
Fair Value, Investments, Entities That Are Calculated Using Net Asset Value Per Share [Line Items]    
Unfunded Commitments $ 196 $ 203