Investments - Summary of Aggregate Contractual or Notional Amounts and Gross Estimated Fair Values Related to Derivative Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Embedded Derivative on Funds Withheld Liability [Member]
Derivative [Line Items]
Contractual/Notional Amount	$ 184	$ 184
Estimated Fair Value, (Liability)	(5)	(3)
With Hedge Designation [Member] | Foreign Exchange, Currency Forwards - Short [Member]
Derivative [Line Items]
Contractual/Notional Amount	34	70
Estimated Fair Value, (Liability)	(3)	(5)
Without Hedge Designation [Member] | Foreign Exchange, Currency Forwards - Short [Member]
Derivative [Line Items]
Contractual/Notional Amount	99	88
Estimated Fair Value, Asset	2	2
Without Hedge Designation [Member] | Equity Markets, Options - Purchased [Member]
Derivative [Line Items]
Contractual/Notional Amount	2,306	544
Estimated Fair Value, Asset	21	24
Without Hedge Designation [Member] | Equity Markets, Options - Written [Member]
Derivative [Line Items]
Contractual/Notional Amount	1,493	292
Estimated Fair Value, (Liability)	(13)	(21)
Without Hedge Designation [Member] | Equity Swaps and Warrants - Long [Member]
Derivative [Line Items]
Contractual/Notional Amount	10	10
Estimated Fair Value, Asset	2	2
Without Hedge Designation [Member] | Equity Futures - Short [Member]
Derivative [Line Items]
Contractual/Notional Amount	399	130
Estimated Fair Value, Asset	3	2
Without Hedge Designation [Member] | Foreign Exchange, Currency Forwards - Long [Member]
Derivative [Line Items]
Contractual/Notional Amount	134	109
Estimated Fair Value, Asset	1
Estimated Fair Value, (Liability)		(3)
Without Hedge Designation [Member] | Currency Options - Long [Member]
Derivative [Line Items]
Contractual/Notional Amount	179	151
Estimated Fair Value, Asset	25	7
Without Hedge Designation [Member] | Currency Options - Short [Member]
Derivative [Line Items]
Contractual/Notional Amount	218
Estimated Fair Value, (Liability)	$ (5)