Investments - Summary of Aggregate Contractual or Notional Amounts and Gross Estimated Fair Values Related to Derivative Financial Instruments (Detail) - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Commodities Swaps [Member] | Short [Member]
Derivative [Line Items]
Contractual/Notional Amount	$ 50,000,000
Embedded Derivative on Funds Withheld Liability [Member]
Derivative [Line Items]
Contractual/Notional Amount	177,000,000	$ 179,000,000
Estimated Fair Value, Asset		5,000,000
Estimated Fair Value, (Liability)	(8,000,000)
Without Hedge Designation [Member] | Equity Markets, Options - Purchased [Member]
Derivative [Line Items]
Contractual/Notional Amount	229,000,000	501,000,000
Estimated Fair Value, Asset	24,000,000	16,000,000
Without Hedge Designation [Member] | Equity Markets, Options - Written [Member]
Derivative [Line Items]
Contractual/Notional Amount	198,000,000	614,000,000
Estimated Fair Value, (Liability)	(10,000,000)	(28,000,000)
Without Hedge Designation [Member] | Equity Futures [Member] | Long [Member]
Derivative [Line Items]
Contractual/Notional Amount		312,000,000
Estimated Fair Value, (Liability)		(1,000,000)
Without Hedge Designation [Member] | Equity Futures [Member] | Short [Member]
Derivative [Line Items]
Contractual/Notional Amount	51,000,000
Estimated Fair Value, (Liability)	(1,000,000)
Without Hedge Designation [Member] | Interest Rate Futures [Member] | Long [Member]
Derivative [Line Items]
Contractual/Notional Amount		63,000,000
Without Hedge Designation [Member] | Currency Forwards [Member] | Long [Member]
Derivative [Line Items]
Contractual/Notional Amount		133,000,000
Estimated Fair Value, Asset		2,000,000
Without Hedge Designation [Member] | Currency Forwards [Member] | Short [Member]
Derivative [Line Items]
Contractual/Notional Amount		152,000,000
Without Hedge Designation [Member] | Currency Options [Member] | Long [Member]
Derivative [Line Items]
Contractual/Notional Amount	250,000,000	550,000,000
Estimated Fair Value, Asset	1,000,000	$ 7,000,000
Without Hedge Designation [Member] | Commodities Futures [Member] | Long [Member]
Derivative [Line Items]
Contractual/Notional Amount	62,000,000
Estimated Fair Value, (Liability)	$ (1,000,000)