Investments (Credit Quality Indicators) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
commercial mortgage and other loans	$ 38,314	$ 37,002
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	16,341	15,089
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	18,017	16,724
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	1,676	1,635
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	9,502	9,263
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	9,714	9,476
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	212	213
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	4,484	3,689
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	4,484	3,689
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	0	0
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	190	219
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	190	219
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	0	0
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan To Value Ratio Total [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	30,517	28,260
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan To Value Ratio Total [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	32,405	30,108
Debt Service Coverage Ratio, Greater than One Point Two X [Member] | Loan To Value Ratio Total [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	1,888	1,848
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	696	487
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	870	673
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	174	186
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	442	801
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	442	801
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	0	0
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	728	776
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	728	776
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	0	0
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	322	770
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	322	770
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	0	0
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan To Value Ratio Total [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	2,188	2,834
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan To Value Ratio Total [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	2,362	3,020
Debt Service Coverage Ratio, One Point Zero X to Less Than Two Point Two X [Member] | Loan To Value Ratio Total [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	174	186
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	224	188
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	268	232
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	44	44
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	265	36
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	265	36
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	0	0
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	180	217
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	180	217
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	0	0
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	542	579
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	588	623
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	46	44
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan To Value Ratio Total [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	1,211	1,020
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan To Value Ratio Total [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	1,301	1,108
Debt Service Coverage Ratio, Less Than One Point Zero X [Member] | Loan To Value Ratio Total [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	90	88
Debt Service Coverage Ratio Total [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	17,261	15,764
Debt Service Coverage Ratio Total [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	19,155	17,629
Debt Service Coverage Ratio Total [Member] | Loan To Value Ratio Less Than Fifty Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	1,894	1,865
Debt Service Coverage Ratio Total [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	10,209	10,100
Debt Service Coverage Ratio Total [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	10,421	10,313
Debt Service Coverage Ratio Total [Member] | Loan-to-Value Ratio, Sixty Percent To Sixty Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	212	213
Debt Service Coverage Ratio Total [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	5,392	4,682
Debt Service Coverage Ratio Total [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	5,392	4,682
Debt Service Coverage Ratio Total [Member] | Loan To Value Ratio Seventy Percent To Seventy Nine Point Nine Nine Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	0	0
Debt Service Coverage Ratio Total [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	1,054	1,568
Debt Service Coverage Ratio Total [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	1,100	1,612
Debt Service Coverage Ratio Total [Member] | Loan-to-Value Ratio, Greater than Eighty Percent [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	46	44
Debt Service Coverage Ratio Total [Member] | Loan To Value Ratio Total [Member] | Commercial Mortgage Loan [Member]
commercial mortgage and other loans	33,916	32,114
Debt Service Coverage Ratio Total [Member] | Loan To Value Ratio Total [Member] | Commercial Mortgage and Agricultural Loans [Member]
commercial mortgage and other loans	36,068	34,236
Debt Service Coverage Ratio Total [Member] | Loan To Value Ratio Total [Member] | Agricultural Loan [Member]
commercial mortgage and other loans	$ 2,152	$ 2,122