Investments (Investment Securities Whose Book Value Exceeded Market Value By More Than 5%) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of Securities	149
Book Value	$ 2,251
Fair Value	1,828
5% To Less Than 16% [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of Securities	23
Book Value	1,073
Fair Value	974
Percentage Book Value Exceeded Market Value, Minimum	5.00%
Percentage Book Value Exceeded Market Value, Maximum	16.00%
16% To Less Than 26% [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of Securities	26
Book Value	796
Fair Value	656
Percentage Book Value Exceeded Market Value, Minimum	16.00%
Percentage Book Value Exceeded Market Value, Maximum	26.00%
26% To 50% [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of Securities	64
Book Value	275
Fair Value	175
Percentage Book Value Exceeded Market Value, Minimum	26.00%
Percentage Book Value Exceeded Market Value, Maximum	50.00%
Greater Than 50% [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of Securities	36
Book Value	107
Fair Value	$ 23
Percentage Book Value Exceeded Market Value, Minimum	50.00%