Investments - Held-to-Maturity Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,825	$ 1,288
Fair Value	$ 5,964	$ 1,317
Weighted Average Yield	2.27%	2.29%
U.S. government obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,068	$ 498
Fair Value	$ 5,189	$ 511
Weighted Average Yield	2.35%	3.08%
U.S. government obligations [Member] | Less than 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,534	$ 498
Fair Value	$ 3,613	$ 511
Weighted Average Yield	2.40%	3.08%
U.S. government obligations [Member] | Greater than 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,534
Fair Value	$ 1,576
Weighted Average Yield	2.25%
Securities backed by real estate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 757	$ 790
Fair Value	$ 775	$ 806
Weighted Average Yield	1.69%	1.80%
Securities backed by real estate [Member] | Less than 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 6	$ 5
Fair Value	$ 6	$ 6
Weighted Average Yield	4.16%	4.61%
Securities backed by real estate [Member] | Greater than 5 years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 751	$ 785
Fair Value	$ 769	$ 800
Weighted Average Yield	1.67%	1.78%