Investments - Cash Instruments by Level (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	$ 74,714	$ 57,827
Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in funds at NAV	3,517	4,206
Investment at fair value	71,197	53,621
Level 1 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	40,116	20,125
Level 2 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	13,807	18,214
Level 3 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	17,274	15,282
U.S. [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	38,444	19,094
U.S. [Member] | Level 1 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	38,444	19,094
Non-U.S. [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	702	36
Non-U.S. [Member] | Level 1 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	640
Non-U.S. [Member] | Level 2 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	62	36
Corporate debt securities [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	10,720	10,838
Corporate debt securities [Member] | Level 1 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	88	48
Corporate debt securities [Member] | Level 2 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	4,781	7,325
Corporate debt securities [Member] | Level 3 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	5,851	3,465
Securities backed by real estate [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	1,870	2,619
Securities backed by real estate [Member] | Level 2 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	940	2,024
Securities backed by real estate [Member] | Level 3 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	930	595
Money market instruments [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	2,197	1,681
Money market instruments [Member] | Level 1 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	771	732
Money market instruments [Member] | Level 2 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	1,426	949
Other debt obligations [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	459	413
Other debt obligations [Member] | Level 2 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	4	94
Other debt obligations [Member] | Level 3 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	455	319
Equity Securities [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	16,805	18,940
Equity Securities [Member] | Level 1 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	173	251
Equity Securities [Member] | Level 2 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	6,594	7,786
Equity Securities [Member] | Level 3 [Member] | Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	$ 10,038	$ 10,903