Investments - Securities Accounted for As Available-for-Sale Included in Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 38,357	$ 19,037
Fair Value	$ 39,079	$ 19,094
Weighted Average Yield	0.84%	1.68%
U.S. [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 37,722	$ 19,037
Fair Value	$ 38,444	$ 19,094
Weighted Average Yield	0.84%	1.68%
Non-U.S. [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 635
Fair Value	$ 635
Weighted Average Yield	0.18%
Less than 5 years [Member] | U.S. [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 29,352	$ 14,063
Fair Value	$ 29,723	$ 14,041
Weighted Average Yield	0.79%	1.53%
Greater than 5 years [Member] | U.S. [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 8,370	$ 4,974
Fair Value	$ 8,721	$ 5,053
Weighted Average Yield	1.09%	2.10%
Greater than 5 years [Member] | Non-U.S. [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 635
Fair Value	$ 635
Weighted Average Yield	0.18%