Investments - Fair Value Measurement Inputs (Detail) $ in Millions	Mar. 31, 2020 USD ($) yr	Dec. 31, 2019 USD ($) yr
Level 3 assets | $	$ 614,112	$ 539,568
Level 3 assets [Member]
Level 3 assets | $	30,776	23,068
Corporate debt securities [Member] | Level 3 assets [Member] | Investments [Member]
Level 3 assets | $	$ 7,149	$ 3,465
Corporate debt securities [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.252	0.298
Corporate debt securities [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.046	0.055
Corporate debt securities [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.118	0.12
Corporate debt securities [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	1.000	1.000
Corporate debt securities [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.400	0.250
Corporate debt securities [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.658	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	1.5	2.9
Corporate debt securities [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	4.0	5.0
Corporate debt securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	39.7	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 | $	$ 775	$ 595
Securities backed by real estate [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.203	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	0.158	0.16
Securities backed by real estate [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.350	0.344
Securities backed by real estate [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.250	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	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	0.4	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 | $	$ 11,056	$ 10,903
Equity Securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	23.8	36.0
Equity Securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.5	0.8
Equity Securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	8.0	8.0
Equity Securities [Member] | Level 3 assets [Member] | Discount rate/yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.203	0.203
Equity Securities [Member] | Level 3 assets [Member] | Discount rate/yield [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.049	0.021
Equity Securities [Member] | Level 3 assets [Member] | Discount rate/yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.14	0.134
Equity Securities [Member] | Level 3 assets [Member] | Capitalization rate [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.160	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	0.059	0.61
Other debt obligations [Member] | Level 3 assets [Member] | Investments [Member]
Level 3 assets | $	$ 428	$ 319
Other debt obligations [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.060	0.052
Other debt obligations [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.032	0.034
Other debt obligations [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.042	0.045
Other debt obligations [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	9.8	8.0
Other debt obligations [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	3.7	4.0
Other debt obligations [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	6.8	6.7