Investments (Details 6) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Investment in limited partnerships
Asset recorded	$ 1,217,000,000	$ 1,290,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	32,000,000	51,000,000
Income (loss) recorded	99,000,000	98,000,000
Per the limited partner financial statements
Total partnership assets	53,000,000,000	54,000,000,000
Total partnership liabilities	6,000,000,000	6,000,000,000
Total partnership valuation adjustment gains	2,000,000,000	2,000,000,000
Total partnership net income	5,000,000,000	3,000,000,000
Indemnity
Investment in limited partnerships
Number of partnerships	62	62
Asset recorded	180,000,000	208,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	5,000,000	11,000,000
Income (loss) recorded	10,000,000	15,000,000
Indemnity | Private equity
Investment in limited partnerships
Number of partnerships	29	29
Asset recorded	73,000,000	82,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	1,000,000	2,000,000
Income (loss) recorded	6,000,000	8,000,000
Indemnity | Private equity | Less than 10%
Investment in limited partnerships
Number of partnerships	26	26
Asset recorded	60,000,000	73,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(3,000,000)	2,000,000
Income (loss) recorded	6,000,000	5,000,000
Indemnity | Private equity | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	13,000,000	9,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	4,000,000	0
Income (loss) recorded	0	3,000,000
Indemnity | Private equity | Greater than 50%
Investment in limited partnerships
Number of partnerships	0	0
Asset recorded	0	0
Income (loss) recognized due to valuation adjustments by the partnerships	0	0
Income (loss) recorded	0	0
Indemnity | Mezzanine debt
Investment in limited partnerships
Number of partnerships	15	15
Asset recorded	27,000,000	35,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(1,000,000)	0
Income (loss) recorded	6,000,000	7,000,000
Indemnity | Mezzanine debt | Less than 10%
Investment in limited partnerships
Number of partnerships	11	11
Asset recorded	18,000,000	22,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(2,000,000)	0
Income (loss) recorded	5,000,000	6,000,000
Indemnity | Mezzanine debt | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	9,000,000	12,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	0	1,000,000
Income (loss) recorded	2,000,000	1,000,000
Indemnity | Mezzanine debt | Greater than 50%
Investment in limited partnerships
Number of partnerships	1	1
Asset recorded	0	1,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	1,000,000	(1,000,000)
Income (loss) recorded	(1,000,000)	0
Indemnity | Real estate
Investment in limited partnerships
Number of partnerships	18	18
Asset recorded	80,000,000	91,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	5,000,000	9,000,000
Income (loss) recorded	(2,000,000)	0
Indemnity | Real estate | Less than 10%
Investment in limited partnerships
Number of partnerships	12	12
Asset recorded	55,000,000	62,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	4,000,000	5,000,000
Income (loss) recorded	(3,000,000)	(1,000,000)
Indemnity | Real estate | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	16,000,000	18,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(1,000,000)	1,000,000
Income (loss) recorded	1,000,000	0
Indemnity | Real estate | Greater than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	9,000,000	11,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	2,000,000	3,000,000
Income (loss) recorded	0	1,000,000
Exchange
Investment in limited partnerships
Number of partnerships	100	100
Asset recorded	1,037,000,000	1,082,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	27,000,000	40,000,000
Income (loss) recorded	89,000,000	83,000,000
Exchange | Private equity
Investment in limited partnerships
Number of partnerships	45	44
Asset recorded	482,000,000	495,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	38,000,000	12,000,000
Income (loss) recorded	23,000,000	42,000,000
Exchange | Private equity | Less than 10%
Investment in limited partnerships
Number of partnerships	42	41
Asset recorded	424,000,000	452,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	22,000,000	13,000,000
Income (loss) recorded	24,000,000	30,000,000
Exchange | Private equity | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	58,000,000	43,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	16,000,000	(1,000,000)
Income (loss) recorded	(1,000,000)	12,000,000
Exchange | Private equity | Greater than 50%
Investment in limited partnerships
Number of partnerships	0	0
Asset recorded	0	0
Income (loss) recognized due to valuation adjustments by the partnerships	0	0
Income (loss) recorded	0	0
Exchange | Mezzanine debt
Investment in limited partnerships
Number of partnerships	25	23
Asset recorded	196,000,000	201,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(6,000,000)	(8,000,000)
Income (loss) recorded	38,000,000	32,000,000
Exchange | Mezzanine debt | Less than 10%
Investment in limited partnerships
Number of partnerships	18	17
Asset recorded	132,000,000	133,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(5,000,000)	(9,000,000)
Income (loss) recorded	29,000,000	26,000,000
Exchange | Mezzanine debt | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	4	3
Asset recorded	27,000,000	33,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	1,000,000	3,000,000
Income (loss) recorded	4,000,000	3,000,000
Exchange | Mezzanine debt | Greater than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	37,000,000	35,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(2,000,000)	(2,000,000)
Income (loss) recorded	5,000,000	3,000,000
Exchange | Real estate
Investment in limited partnerships
Number of partnerships	30	33
Asset recorded	359,000,000	386,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(5,000,000)	36,000,000
Income (loss) recorded	28,000,000	9,000,000
Exchange | Real estate | Less than 10%
Investment in limited partnerships
Number of partnerships	22	25
Asset recorded	274,000,000	284,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(7,000,000)	31,000,000
Income (loss) recorded	26,000,000	(1,000,000)
Exchange | Real estate | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	5	5
Asset recorded	52,000,000	59,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(4,000,000)	3,000,000
Income (loss) recorded	3,000,000	0
Exchange | Real estate | Greater than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	33,000,000	43,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	6,000,000	2,000,000
Income (loss) recorded	$ (1,000,000)	$ 10,000,000