Investments (Details 6) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Investment in limited partnerships
Asset recorded	$ 1,290,000,000	$ 1,324,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	51,000,000	120,000,000
Income (loss) recorded	98,000,000	8,000,000
Per the limited partner financial statements
Total partnership assets	54,000,000,000	58,000,000,000
Total partnership liabilities	6,000,000,000	10,000,000,000
Total partnership valuation adjustment gains	2,000,000,000	4,000,000,000
Total partnership net income	3,000,000,000	3,000,000,000
Indemnity
Investment in limited partnerships
Number of partnerships	62	64
Asset recorded	208,000,000	216,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	11,000,000	57,000,000
Income (loss) recorded	15,000,000	(36,000,000)
Indemnity | Private equity
Investment in limited partnerships
Number of partnerships	29	29
Asset recorded	82,000,000	86,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	2,000,000	7,000,000
Income (loss) recorded	8,000,000	7,000,000
Indemnity | Private equity | Less than 10%
Investment in limited partnerships
Number of partnerships	26	26
Asset recorded	73,000,000	78,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	2,000,000	4,000,000
Income (loss) recorded	5,000,000	7,000,000
Indemnity | Private equity | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	9,000,000	8,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	0	3,000,000
Income (loss) recorded	3,000,000	0
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	35,000,000	47,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	0	6,000,000
Income (loss) recorded	7,000,000	1,000,000
Indemnity | Mezzanine debt | Less than 10%
Investment in limited partnerships
Number of partnerships	11	11
Asset recorded	22,000,000	30,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	0	4,000,000
Income (loss) recorded	6,000,000	3,000,000
Indemnity | Mezzanine debt | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	12,000,000	15,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	1,000,000	2,000,000
Income (loss) recorded	1,000,000	(2,000,000)
Indemnity | Mezzanine debt | Greater than 50%
Investment in limited partnerships
Number of partnerships	1	1
Asset recorded	1,000,000	2,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(1,000,000)	0
Income (loss) recorded	0	0
Indemnity | Real estate
Investment in limited partnerships
Number of partnerships	18	20
Asset recorded	91,000,000	83,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	9,000,000	44,000,000
Income (loss) recorded	0	(44,000,000)
Indemnity | Real estate | Less than 10%
Investment in limited partnerships
Number of partnerships	12	12
Asset recorded	62,000,000	59,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	5,000,000	30,000,000
Income (loss) recorded	(1,000,000)	(31,000,000)
Indemnity | Real estate | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	4
Asset recorded	18,000,000	14,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	1,000,000	10,000,000
Income (loss) recorded	0	(10,000,000)
Indemnity | Real estate | Greater than 50%
Investment in limited partnerships
Number of partnerships	3	4
Asset recorded	11,000,000	10,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	3,000,000	4,000,000
Income (loss) recorded	1,000,000	(3,000,000)
Exchange
Investment in limited partnerships
Number of partnerships	100	99
Asset recorded	1,082,000,000	1,108,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	40,000,000	63,000,000
Income (loss) recorded	83,000,000	44,000,000
Exchange | Private equity
Investment in limited partnerships
Number of partnerships	44	44
Asset recorded	495,000,000	555,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	12,000,000	38,000,000
Income (loss) recorded	42,000,000	39,000,000
Exchange | Private equity | Less than 10%
Investment in limited partnerships
Number of partnerships	41	41
Asset recorded	452,000,000	517,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	13,000,000	28,000,000
Income (loss) recorded	30,000,000	40,000,000
Exchange | Private equity | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	43,000,000	38,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(1,000,000)	10,000,000
Income (loss) recorded	12,000,000	0
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	(1,000,000)
Exchange | Mezzanine debt
Investment in limited partnerships
Number of partnerships	23	20
Asset recorded	201,000,000	214,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(8,000,000)	14,000,000
Income (loss) recorded	32,000,000	13,000,000
Exchange | Mezzanine debt | Less than 10%
Investment in limited partnerships
Number of partnerships	17	14
Asset recorded	133,000,000	142,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(9,000,000)	12,000,000
Income (loss) recorded	26,000,000	13,000,000
Exchange | Mezzanine debt | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	33,000,000	41,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	3,000,000	2,000,000
Income (loss) recorded	3,000,000	(2,000,000)
Exchange | Mezzanine debt | Greater than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	35,000,000	31,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(2,000,000)	0
Income (loss) recorded	3,000,000	2,000,000
Exchange | Real estate
Investment in limited partnerships
Number of partnerships	33	35
Asset recorded	386,000,000	339,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	36,000,000	11,000,000
Income (loss) recorded	9,000,000	(8,000,000)
Exchange | Real estate | Less than 10%
Investment in limited partnerships
Number of partnerships	25	25
Asset recorded	284,000,000	250,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	31,000,000	(11,000,000)
Income (loss) recorded	(1,000,000)	10,000,000
Exchange | Real estate | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	5	6
Asset recorded	59,000,000	52,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	3,000,000	7,000,000
Income (loss) recorded	0	(7,000,000)
Exchange | Real estate | Greater than 50%
Investment in limited partnerships
Number of partnerships	3	4
Asset recorded	43,000,000	37,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	2,000,000	15,000,000
Income (loss) recorded	$ 10,000,000	$ (11,000,000)