Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3		$ 0		$ 7,200,000	$ 5,400,000
Gross transfers into Level 3		$ 800,000	$ 5,700,000	5,500,000	8,400,000
Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3			$ 5,100,000		5,400,000
Debt Securities | U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3				4,800,000	5,100,000
Gross transfers into Level 3				3,800,000	5,500,000
Debt Securities | Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3				300,000	1,900,000
Equity Securities | Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3				2,400,000
Gross transfers into Level 3				$ 1,400,000	1,000,000
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3	[1]				$ 300,000

[1] Includes partnership and non-marketable equity investments accounted for on an AFS basis.