Investments - Summary of Aggregate Contractual or Notional Amounts and Estimated Fair Values Related to Derivative Financial Instruments (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Embedded Derivative on Funds Withheld Liability [Member]
Derivative [Line Items]
Contractual/Notional Amount	$ 167,000,000	$ 174,000,000
Estimated Fair Value, Asset		3,000,000
Estimated Fair Value, (Liability)	(3,000,000)
With Hedge Designation [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Contractual/Notional Amount	500,000,000
Estimated Fair Value, Asset	4,000,000
Without Hedge Designation [Member] | Equity Markets, Options - Purchased [Member]
Derivative [Line Items]
Contractual/Notional Amount	224,000,000	223,000,000
Estimated Fair Value, Asset	12,000,000	14,000,000
Without Hedge Designation [Member] | Equity Markets, Options - Written [Member]
Derivative [Line Items]
Contractual/Notional Amount	290,000,000	267,000,000
Estimated Fair Value, (Liability)	(7,000,000)	(8,000,000)
Without Hedge Designation [Member] | Equity Futures [Member] | Short [Member]
Derivative [Line Items]
Contractual/Notional Amount	265,000,000	225,000,000
Estimated Fair Value, Asset	1,000,000	1,000,000
Without Hedge Designation [Member] | Commodity Futures [Member] | Long [Member]
Derivative [Line Items]
Contractual/Notional Amount	$ 44,000,000	$ 42,000,000