Investments - Fair Value Measurement Inputs (Detail) $ in Millions	Dec. 31, 2019 USD ($) yr	Dec. 31, 2018 USD ($) yr
Level 3 assets | $	$ 539,568	$ 501,712
Level 3 assets [Member]
Level 3 assets | $	23,068	22,181
Corporate debt securities [Member] | Level 3 assets [Member] | Investments [Member]
Level 3 assets | $	$ 3,465	$ 2,540
Corporate debt securities [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.298	0.323
Corporate debt securities [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.055	0.065
Corporate debt securities [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.12	0.127
Corporate debt securities [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	1.000	0.780
Corporate debt securities [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.250	0.505
Corporate debt securities [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.685	0.746
Corporate debt securities [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	5.9	4.7
Corporate debt securities [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	2.9	2.0
Corporate debt securities [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	5.0	3.8
Corporate debt securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	24.4	20.3
Corporate debt securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.6	1.3
Corporate debt securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	7.0	7.7
Securities backed by real estate [Member] | Level 3 assets [Member] | Investments [Member]
Level 3 assets | $	$ 595	$ 457
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	0.094	0.095
Securities backed by real estate [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.16	0.155
Securities backed by real estate [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.344	0.397
Securities backed by real estate [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.331	0.329
Securities backed by real estate [Member] | Level 3 assets [Member] | Recovery rate [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.335	0.379
Securities backed by real estate [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	3.0	4.8
Securities backed by real estate [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.4	1.1
Securities backed by real estate [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.9	(2.2)
Equity Securities [Member] | Level 3 assets [Member] | Investments [Member]
Level 3 assets | $	$ 10,903	$ 10,335
Equity Securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	36.0	23.6
Equity Securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.8	1.0
Equity Securities [Member] | Level 3 assets [Member] | Multiples [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	8.0	8.1
Equity Securities [Member] | Level 3 assets [Member] | Discount rate/yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.203	0.221
Equity Securities [Member] | Level 3 assets [Member] | Discount rate/yield [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.021	0.072
Equity Securities [Member] | Level 3 assets [Member] | Discount rate/yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.134	0.144
Equity Securities [Member] | Level 3 assets [Member] | Capitalization rate [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.151	0.123
Equity Securities [Member] | Level 3 assets [Member] | Capitalization rate [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.036	0.035
Equity Securities [Member] | Level 3 assets [Member] | Capitalization rate [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.061	0.061
Other debt obligations [Member] | Level 3 assets [Member] | Investments [Member]
Level 3 assets | $	$ 319	$ 216
Other debt obligations [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	0.052	0.061
Other debt obligations [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	0.034	0.041
Other debt obligations [Member] | Level 3 assets [Member] | Yield [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	0.045	0.052
Other debt obligations [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Maximum [Member]
Investments measurement input	8.0	9.0
Other debt obligations [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Minimum [Member]
Investments measurement input	4.0	4.0
Other debt obligations [Member] | Level 3 assets [Member] | Duration [Member] | Investments [Member] | Weighted Average [Member]
Investments measurement input	6.7	7.5