Investments (Details 10) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Contracts	Dec. 31, 2011 Contracts
Financing Receivable, Modifications [Line Items]
Number of Mortgage Loans	6	15
Carrying Value after Specific Valuation Allowance, Pre-Modification	$ 239	$ 189
Carrying Value after Specific Valuation Allowance, Post-Modification	215	153
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Mortgage Loans	1	5
Carrying Value after Specific Valuation Allowance, Pre-Modification	222	147
Carrying Value after Specific Valuation Allowance, Post-Modification	199	111
Agricultural [Member]
Financing Receivable, Modifications [Line Items]
Number of Mortgage Loans	5	10
Carrying Value after Specific Valuation Allowance, Pre-Modification	17	42
Carrying Value after Specific Valuation Allowance, Post-Modification	16	42
Residential [Member]
Financing Receivable, Modifications [Line Items]
Number of Mortgage Loans	0	0
Carrying Value after Specific Valuation Allowance, Pre-Modification	0	0
Carrying Value after Specific Valuation Allowance, Post-Modification	$ 0	$ 0