Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Finance receivable	$ 86	$ 77
Loss and loss expense reserves	2,900	2,835
Life Policy Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Finance receivable	33	32
Investment contract reserves:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loss and loss expense reserves	1,562	1,559
Investment contract reserves: | Deferred annuities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loss and loss expense reserves	760	760
Investment contract reserves: | Structured Settlement Annuity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loss and loss expense reserves	147	151
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash and Cash Equivalents, Fair Value Disclosure	$ 914	$ 767