Investments - Held-to-Maturity Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,756	$ 5,825
Fair Value	$ 6,124	$ 5,964
Weighted Average Yield	2.27%	2.27%
U.S. government obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,055	$ 5,068
Fair Value	$ 5,431	$ 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	$ 3,531	$ 3,534
Fair Value	$ 3,751	$ 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	$ 1,524	$ 1,534
Fair Value	$ 1,680	$ 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	$ 701	$ 757
Fair Value	$ 693	$ 775
Weighted Average Yield	1.64%	1.69%
Securities backed by real estate [Member] | Less than 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5	$ 6
Fair Value	$ 5	$ 6
Weighted Average Yield	3.91%	4.16%
Securities backed by real estate [Member] | Greater than 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 696	$ 751
Fair Value	$ 688	$ 769
Weighted Average Yield	1.62%	1.67%