Fair Value Measurements - Information about Valuation Inputs for Fixed Income Securities Classified as Level 3 (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	$ 5,884	$ 7,139	$ 6,207	$ 6,930
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	130	160
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	(377)	135
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(76)	(87)
Derivative [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	77	53	66	46
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	12	8
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(1)	(1)
Credit Derivative [Member] | Credit Derivative [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	(2)	(1)	0	(1)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	(1)	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	0	0
Interest Rate Swap [Member] | Interest Rate Swap [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	79	54	$ 67	$ 47
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	13	8
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	$ (1)	$ (1)