Fair Value Measurements (Fair Value And Maximum Future Funding Commitments Related To Investments) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Fair Value [Member]
Fair Value And Maximum Future Funding Commitments Related To Investments [Line Items]
Financial	237	225
Real estate	342	292
Distressed	182	192
Mezzanine	274	284
Traditional	737	711
Vintage	14	14
Investment funds	404	395
Total of alternative investments	2,190	2,113
Maximum Future Funding Commitments [Member]
Fair Value And Maximum Future Funding Commitments Related To Investments [Line Items]
Financial	106	111
Real estate	67	62
Distressed	147	152
Mezzanine	282	279
Traditional	575	587
Vintage	0	0
Total of alternative investments	1,177	1,191
Expected Liquidation Period Lower Range [Member]
Fair Value And Maximum Future Funding Commitments Related To Investments [Line Items]
Liquidation period for financial alternative investments (in years)	5 years	5 years
Liquidation period for real estate alternative investments (in years)	3 years	3 years
Liquidation period for distressed alternative investments (in years)	6 years	6 years
Liquidation period for mezzanine alternative investments (in years)	6 years	6 years
Liquidation period for traditional alternative investments (in years)	3 years	3 years
Liquidation period for vintage alternative investments (in years)	1 year	1 year
Expected Liquidation Period Upper Range [Member]
Fair Value And Maximum Future Funding Commitments Related To Investments [Line Items]
Liquidation period for financial alternative investments (in years)	9 years	9 years
Liquidation period for real estate alternative investments (in years)	9 years	9 years
Liquidation period for distressed alternative investments (in years)	9 years	9 years
Liquidation period for mezzanine alternative investments (in years)	9 years	9 years
Liquidation period for traditional alternative investments (in years)	8 years	8 years
Liquidation period for vintage alternative investments (in years)	3 years	3 years