Investments - Held-to-Maturity Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,205	$ 5,301
Fair Value	$ 5,438	$ 5,637
Weighted Average Yield	2.16%	2.15%
U.S. government obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,542	$ 4,561
Fair Value	$ 4,749	$ 4,883
Weighted Average Yield	2.33%	2.33%
U.S. government obligations [Member] \| Less than 1 year [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 504	$ 501
Fair Value	$ 507	$ 513
Weighted Average Yield	2.55%	2.53%
U.S. government obligations [Member] \| 1 year to 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,038	$ 2,529
Fair Value	$ 4,242	$ 2,695
Weighted Average Yield	2.30%	2.34%
U.S. government obligations [Member] \| 5 years to 10 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		$ 1,531
Fair Value		$ 1,675
Weighted Average Yield		2.25%
Securities backed by real estate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 663	$ 740
Fair Value	$ 689	$ 754
Weighted Average Yield	1.02%	1.08%
Securities backed by real estate [Member] \| 5 years to 10 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3	$ 4
Fair Value	$ 3	$ 3
Weighted Average Yield	2.64%	2.56%
Securities backed by real estate [Member] \| Greater than 10 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 660	$ 736
Fair Value	$ 686	$ 751
Weighted Average Yield	1.00%	1.08%