Investments - Held-to-Maturity Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,301	$ 5,825
Fair Value	$ 5,637	$ 5,964
Weighted Average Yield	2.15%	2.27%
U.S. government obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,561	$ 5,068
Fair Value	$ 4,883	$ 5,189
Weighted Average Yield	2.33%	2.35%
U.S. government obligations [Member] \| Less than 1 year [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 501
Fair Value	$ 513
Weighted Average Yield	2.53%
U.S. government obligations [Member] \| 1 year to 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,529	$ 3,534
Fair Value	$ 2,695	$ 3,613
Weighted Average Yield	2.34%	2.40%
U.S. government obligations [Member] \| 5 years to 10 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,531	$ 1,534
Fair Value	$ 1,675	$ 1,576
Weighted Average Yield	2.25%	2.25%
Securities backed by real estate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 740	$ 757
Fair Value	$ 754	$ 775
Weighted Average Yield	1.08%	1.69%
Securities backed by real estate [Member] \| Less than 1 year [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		$ 6
Fair Value		$ 6
Weighted Average Yield		4.16%
Securities backed by real estate [Member] \| 5 years to 10 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4
Fair Value	$ 3
Weighted Average Yield	2.56%
Securities backed by real estate [Member] \| Greater than 10 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 736	$ 751
Fair Value	$ 751	$ 769
Weighted Average Yield	1.08%	1.67%