Investments - Held-to-Maturity Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,771	$ 5,825
Fair Value	$ 6,139	$ 5,964
Weighted Average Yield	2.20%	2.27%
U.S. government obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,052	$ 5,068
Fair Value	$ 5,406	$ 5,189
Weighted Average Yield	2.35%	2.35%
U.S. government obligations [Member] | Less than 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,530	$ 3,534
Fair Value	$ 3,727	$ 3,613
Weighted Average Yield	2.40%	2.40%
U.S. government obligations [Member] | Greater than 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,522	$ 1,534
Fair Value	$ 1,679	$ 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	$ 719	$ 757
Fair Value	$ 733	$ 775
Weighted Average Yield	1.11%	1.69%
Securities backed by real estate [Member] | Less than 5 years [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] | Greater than 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 719	$ 751
Fair Value	$ 733	$ 769
Weighted Average Yield	1.11%	1.67%