Investments (Details 9) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Impaired Mortgage Loans
Unpaid Principal Balance	$ 750	$ 593
Valuation allowances	347	481	664	721
Recorded Investment	616	470
Average Investment	681	588
Interest Income	22	11
Loans with a Valuation Allowance [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	568	269
Recorded Investment	556	268
Valuation allowances	117	105
Recorded Investment	439	163
Loans Without Valuation Allowances [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	182	324
Recorded Investment	177	307
Commercial mortgage loans portfolio segment [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	548	348
Valuation allowances	293	398	562	589
Recorded Investment	445	274
Average Investment	464	313
Interest Income	14	6
Commercial mortgage loans portfolio segment [Member] | Loans with a Valuation Allowance [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	445	96
Recorded Investment	436	96
Valuation allowances	94	59
Recorded Investment	342	37
Commercial mortgage loans portfolio segment [Member] | Loans Without Valuation Allowances [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	103	252
Recorded Investment	103	237
Agricultural mortgage loans portfolio segment [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	189	231
Valuation allowances	52	81	88	115
Recorded Investment	160	183
Average Investment	204	252
Interest Income	8	5
Agricultural mortgage loans portfolio segment [Member] | Loans with a Valuation Allowance [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	110	160
Recorded Investment	107	159
Valuation allowances	21	45
Recorded Investment	86	114
Agricultural mortgage loans portfolio segment [Member] | Loans Without Valuation Allowances [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	79	71
Recorded Investment	74	69
Residential mortgage loans portfolio segment [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	13	14
Valuation allowances	2	2	14	17
Recorded Investment	11	13
Average Investment	13	23
Interest Income	0	0
Residential mortgage loans portfolio segment [Member] | Loans with a Valuation Allowance [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	13	13
Recorded Investment	13	13
Valuation allowances	2	1
Recorded Investment	11	12
Residential mortgage loans portfolio segment [Member] | Loans Without Valuation Allowances [Member]
Impaired Mortgage Loans
Unpaid Principal Balance	0	1
Recorded Investment	$ 0	$ 1