Investments (Details 6) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Investment in limited partnerships
Asset recorded	$ 1,171,000,000	$ 1,217,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(22,000,000)	32,000,000
Income (loss) recorded	58,000,000	99,000,000
Per the limited partner financial statements
Total partnership assets	51,000,000,000	53,000,000,000
Total partnership liabilities	5,000,000,000	6,000,000,000
Total partnership valuation adjustment gains (losses)	(1,000,000,000)	2,000,000,000
Total partnership net income	2,000,000,000	5,000,000,000
Indemnity
Investment in limited partnerships
Number of partnerships	62	62
Asset recorded	167,000,000	180,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(3,000,000)	5,000,000
Income (loss) recorded	6,000,000	10,000,000
Indemnity | Private equity
Investment in limited partnerships
Number of partnerships	29	29
Asset recorded	68,000,000	73,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(4,000,000)	1,000,000
Income (loss) recorded	3,000,000	6,000,000
Indemnity | Private equity | Less than 10%
Investment in limited partnerships
Number of partnerships	26	26
Asset recorded	55,000,000	60,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(4,000,000)	(3,000,000)
Income (loss) recorded	3,000,000	6,000,000
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	13,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	0	4,000,000
Income (loss) recorded	0	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	24,000,000	27,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(1,000,000)	(1,000,000)
Income (loss) recorded	2,000,000	6,000,000
Indemnity | Mezzanine debt | Less than 10%
Investment in limited partnerships
Number of partnerships	11	11
Asset recorded	16,000,000	18,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	0	(2,000,000)
Income (loss) recorded	0	5,000,000
Indemnity | Mezzanine debt | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	8,000,000	9,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(1,000,000)	0
Income (loss) recorded	2,000,000	2,000,000
Indemnity | Mezzanine debt | Greater than 50%
Investment in limited partnerships
Number of partnerships	1	1
Asset recorded	0	0
Income (loss) recognized due to valuation adjustments by the partnerships	0	1,000,000
Income (loss) recorded	0	(1,000,000)
Indemnity | Real estate
Investment in limited partnerships
Number of partnerships	18	18
Asset recorded	75,000,000	80,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	2,000,000	5,000,000
Income (loss) recorded	1,000,000	(2,000,000)
Indemnity | Real estate | Less than 10%
Investment in limited partnerships
Number of partnerships	12	12
Asset recorded	52,000,000	55,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	0	4,000,000
Income (loss) recorded	0	(3,000,000)
Indemnity | Real estate | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	14,000,000	16,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	0	(1,000,000)
Income (loss) recorded	1,000,000	1,000,000
Indemnity | Real estate | Greater than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	9,000,000	9,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	2,000,000	2,000,000
Income (loss) recorded	0	0
Exchange
Investment in limited partnerships
Number of partnerships	101	100
Asset recorded	1,004,000,000	1,037,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(19,000,000)	27,000,000
Income (loss) recorded	52,000,000	89,000,000
Exchange | Private equity
Investment in limited partnerships
Number of partnerships	46	45
Asset recorded	476,000,000	482,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(17,000,000)	38,000,000
Income (loss) recorded	27,000,000	23,000,000
Exchange | Private equity | Less than 10%
Investment in limited partnerships
Number of partnerships	43	42
Asset recorded	418,000,000	424,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(18,000,000)	22,000,000
Income (loss) recorded	27,000,000	24,000,000
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	58,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	1,000,000	16,000,000
Income (loss) recorded	0	(1,000,000)
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	25
Asset recorded	185,000,000	196,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(2,000,000)	(6,000,000)
Income (loss) recorded	9,000,000	38,000,000
Exchange | Mezzanine debt | Less than 10%
Investment in limited partnerships
Number of partnerships	18	18
Asset recorded	124,000,000	132,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	0	(5,000,000)
Income (loss) recorded	4,000,000	29,000,000
Exchange | Mezzanine debt | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	4	4
Asset recorded	24,000,000	27,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(3,000,000)	1,000,000
Income (loss) recorded	5,000,000	4,000,000
Exchange | Mezzanine debt | Greater than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	37,000,000	37,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	1,000,000	(2,000,000)
Income (loss) recorded	0	5,000,000
Exchange | Real estate
Investment in limited partnerships
Number of partnerships	30	30
Asset recorded	343,000,000	359,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	0	(5,000,000)
Income (loss) recorded	16,000,000	28,000,000
Exchange | Real estate | Less than 10%
Investment in limited partnerships
Number of partnerships	21	22
Asset recorded	234,000,000	274,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	(6,000,000)	(7,000,000)
Income (loss) recorded	12,000,000	26,000,000
Exchange | Real estate | Greater than or equal to 10% but less than 50%
Investment in limited partnerships
Number of partnerships	6	5
Asset recorded	78,000,000	52,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	1,000,000	(4,000,000)
Income (loss) recorded	4,000,000	3,000,000
Exchange | Real estate | Greater than 50%
Investment in limited partnerships
Number of partnerships	3	3
Asset recorded	31,000,000	33,000,000
Income (loss) recognized due to valuation adjustments by the partnerships	5,000,000	6,000,000
Income (loss) recorded	$ 0	$ (1,000,000)