Investments (Distribution Of Securities By Percentage Of Fair Value Below Book Value By More Than 5%) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 Security
>5% to 15% [Member]
Schedule of Investments [Line Items]
Number of Securities	18
Book Value	$ 741
Fair Value	641
Percentage of Fair Value Below Book Value, Minimum	5.00%
Percentage of Fair Value Below Book Value, Maximum	15.00%
>15% to 25% [Member]
Schedule of Investments [Line Items]
Number of Securities	15
Book Value	136
Fair Value	110
Percentage of Fair Value Below Book Value, Minimum	15.00%
Percentage of Fair Value Below Book Value, Maximum	25.00%
>25% to 50% [Member]
Schedule of Investments [Line Items]
Number of Securities	15
Book Value	56
Fair Value	40
Percentage of Fair Value Below Book Value, Minimum	25.00%
Percentage of Fair Value Below Book Value, Maximum	50.00%
> 50% [Member]
Schedule of Investments [Line Items]
Number of Securities	20
Book Value	80
Fair Value	17
Percentage of Fair Value Below Book Value, Minimum	50.00%
Greater Than 5% [Member]
Schedule of Investments [Line Items]
Number of Securities	68
Book Value	1,013
Fair Value	$ 808