Investments - Held-to-Maturity Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,260	$ 5,301
Fair Value	$ 5,516	$ 5,637
Weighted Average Yield	2.17%	2.15%
U.S. government obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,558	$ 4,561
Fair Value	$ 4,789	$ 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	$ 501	$ 501
Fair Value	$ 506	$ 513
Weighted Average Yield	2.53%	2.53%
U.S. government obligations [Member] \| 1 year to 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,057	$ 2,529
Fair Value	$ 4,283	$ 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	$ 702	$ 740
Fair Value	$ 727	$ 754
Weighted Average Yield	1.12%	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	2.65%	2.56%
Securities backed by real estate [Member] \| Greater than 10 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 699	$ 736
Fair Value	$ 724	$ 751
Weighted Average Yield	1.11%	1.08%