Investments - Fair Value Measurement Inputs (Detail) $ in Millions	Sep. 30, 2020 USD ($) yr	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) yr
Level 3 assets | $	$ 630,012	$ 641,711	$ 539,568
Level 3 assets [Member]
Level 3 assets | $	27,677	$ 29,426	23,068
Corporate debt securities [Member] | Level 3 assets [Member] | Investments [Member]
Level 3 assets | $	$ 5,851		$ 3,465
Corporate debt securities [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.334		0.298
Corporate debt securities [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.047		0.055
Corporate debt securities [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.112		0.12
Corporate debt securities [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.700		1.000
Corporate debt securities [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.350		0.250
Corporate debt securities [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.543		0.685
Corporate debt securities [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	8.0		5.9
Corporate debt securities [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	1.8		2.9
Corporate debt securities [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	4.1		5.0
Corporate debt securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	31.0		24.4
Corporate debt securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.6		0.6
Corporate debt securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	7.4		7.0
Securities backed by real estate [Member] | Level 3 assets [Member] | Investments [Member]
Level 3 assets | $	$ 930		$ 595
Securities backed by real estate [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.537		0.203
Securities backed by real estate [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.100		0.094
Securities backed by real estate [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.187		0.16
Securities backed by real estate [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Maximum [Member]
Investments measurement input			0.344
Securities backed by real estate [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Minimum [Member]
Investments measurement input			0.331
Securities backed by real estate [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input			0.335
Securities backed by real estate [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	3.7		3.0
Securities backed by real estate [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.6		0.4
Securities backed by real estate [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	2.3		0.9
Equity Securities [Member] | Level 3 assets [Member] | Investments [Member]
Level 3 assets | $	$ 10,038		$ 10,903
Equity Securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	26.9		36.0
Equity Securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.8		0.8
Equity Securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	9.9		8.0
Equity Securities [Member] | Level 3 assets [Member] | Discount rate/yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.310		0.203
Equity Securities [Member] | Level 3 assets [Member] | Discount rate/yield [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.040		0.021
Equity Securities [Member] | Level 3 assets [Member] | Discount rate/yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.138		0.134
Equity Securities [Member] | Level 3 assets [Member] | Capitalization rate [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.153		0.151
Equity Securities [Member] | Level 3 assets [Member] | Capitalization rate [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.037		0.036
Equity Securities [Member] | Level 3 assets [Member] | Capitalization rate [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.65		0.61
Other debt obligations [Member] | Level 3 assets [Member] | Investments [Member]
Level 3 assets | $	$ 455		$ 319
Other debt obligations [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.062		0.052
Other debt obligations [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.021		0.034
Other debt obligations [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.034		0.045
Other debt obligations [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	10.3		8.0
Other debt obligations [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.5		4.0
Other debt obligations [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	6.2		6.7