Long-term and Short-term Debt (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Capital lease obligations	$ 33	$ 37
Long-term debt	16,535	20,624
Short-term debt	100	686
Total	16,635	21,310
Senior Notes [Member]
Debt Instrument [Line Items]
Interest Rate Range Minimum	0.79%
Interest Rate Range Maximum	7.72%
Weighted Average Interest Rate	4.83%
Debt Instrument, Maturity Date Range, Start	Aug. 06, 2013
Debt Instrument, Maturity Date Range, End	Jun. 15, 2045
Long-term Debt	15,669	15,666
Debt Advances Agreements [Member]
Debt Instrument [Line Items]
Interest Rate Range Minimum	0.23%
Interest Rate Range Maximum	4.86%
Weighted Average Interest Rate	2.33%
Debt Instrument, Maturity Date Range, Start	Apr. 09, 2012
Debt Instrument, Maturity Date Range, End	May 29, 2012
Long-term Debt	0	4,179
Surplus Notes [Member]
Debt Instrument [Line Items]
Interest Rate Range Minimum	7.63%
Interest Rate Range Maximum	7.88%
Weighted Average Interest Rate	7.84%
Debt Instrument, Maturity Date Range, Start	Nov. 01, 2015
Debt Instrument, Maturity Date Range, End	Nov. 01, 2025
Long-term Debt	700	700
Other Notes [Member]
Debt Instrument [Line Items]
Interest Rate Range Minimum	0.22%
Interest Rate Range Maximum	8.00%
Weighted Average Interest Rate	4.47%
Debt Instrument, Maturity Date Range, Start	Dec. 31, 2016
Debt Instrument, Maturity Date Range, End	Nov. 30, 2030
Long-term Debt	$ 133	$ 42