Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finance receivable	$ 61	$ 61
Loss and loss expense reserves	5,345	5,273
Life Policy Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finance receivable	31	31
Deferred Annuities And Structured Settlements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss and loss expense reserves	821	835
Loss and loss expense reserves	$ 161	$ 161
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage Of Level 3 Assets To Total Financial Instruments Measured At Fair Value	1.00%