Fair Value of Financial Instruments - Narrative (Detail) $ in Millions		3 Months Ended	9 Months Ended
	Nov. 30, 2017	Sep. 30, 2018 USD ($) $ / Contract	Sep. 30, 2018 USD ($) $ / Contract	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		$ 204	$ 229
Assets, net transfer in (out) of Level 3		(146)	(440)
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, net transfer in (out) of Level 3		$ (77)	$ (330)
Income Approach Valuation Technique | Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, strike price | $ / Contract		0	0
Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred Stock, Call Features, Minimum Term	5 years
Preferred Stock, Dividend Rate, Percentage, Less Than Par	90.00%
Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held by insurance regulators		$ 20,705	$ 20,705		$ 20,301
FSRC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Undiscounted Cash flows used in fair value measurement				$ 1,025