Investments - Summary of Aggregate Contractual or Notional Amount and Estimated Fair Value Related to Derivative Financial Instruments (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Embedded Derivative on Funds Withheld Liability [Member]
Derivative [Line Items]
Contractual/Notional Amount	$ 179,000,000	$ 184,000,000
Estimated Fair Value, Asset	5,000,000
Estimated Fair Value, (Liability)		(3,000,000)
With Hedge Designation [Member] | Foreign Exchange, Currency Forwards - Short [Member]
Derivative [Line Items]
Contractual/Notional Amount		70,000,000
Estimated Fair Value, (Liability)		(5,000,000)
Without Hedge Designation [Member] | Foreign Exchange, Currency Forwards - Short [Member]
Derivative [Line Items]
Contractual/Notional Amount	152,000,000	88,000,000
Estimated Fair Value, Asset		2,000,000
Without Hedge Designation [Member] | Equity Markets, Options - Purchased [Member]
Derivative [Line Items]
Contractual/Notional Amount	501,000,000	544,000,000
Estimated Fair Value, Asset	16,000,000	24,000,000
Without Hedge Designation [Member] | Equity Markets, Options - Written [Member]
Derivative [Line Items]
Contractual/Notional Amount	614,000,000	292,000,000
Estimated Fair Value, (Liability)	(28,000,000)	(21,000,000)
Without Hedge Designation [Member] | Equity Futures - Long [Member]
Derivative [Line Items]
Contractual/Notional Amount	312,000,000
Estimated Fair Value, (Liability)	(1,000,000)
Without Hedge Designation [Member] | Equity Futures - Short [Member]
Derivative [Line Items]
Contractual/Notional Amount		130,000,000
Estimated Fair Value, Asset		2,000,000
Without Hedge Designation [Member] | Interest Rate Futures - Long [Member]
Derivative [Line Items]
Contractual/Notional Amount	63,000,000
Without Hedge Designation [Member] | Foreign Exchange, Currency Forwards - Long [Member]
Derivative [Line Items]
Contractual/Notional Amount	133,000,000	109,000,000
Estimated Fair Value, Asset	2,000,000
Estimated Fair Value, (Liability)		(3,000,000)
Without Hedge Designation [Member] | Currency Options - Long [Member]
Derivative [Line Items]
Contractual/Notional Amount	550,000,000	151,000,000
Estimated Fair Value, Asset	$ 7,000,000	$ 7,000,000