Available-for-sale securities (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Investments, Debt and Equity Securities [Abstract]
Charges recognized for "other-than-temporary" declines in the market values of equities securities	$ 2	$ 5	$ 3
Less than 12 months
Fair Value	109	164	38
Unrealized Losses	3	8	1
12 months or more
Fair Value	59	53	70
Unrealized Losses	5	10	11
Total
Fair Value	168	217	108
Unrealized Losses	8	18	12
Corporate bonds
Less than 12 months
Fair Value		54
Unrealized Losses		1
12 months or more
Fair Value		1
Unrealized Losses		0
Total
Fair Value		55
Unrealized Losses		1
Asset-backed securities
Less than 12 months
Fair Value	0	1	19
Unrealized Losses	0	0	0
12 months or more
Fair Value	20	20	19
Unrealized Losses	3	5	4
Total
Fair Value	20	21	38
Unrealized Losses	3	5	4
U.S. governmental agency mortgage-backed securities
Less than 12 months
Fair Value	84	51
Unrealized Losses	1	1
12 months or more
Fair Value	15	0
Unrealized Losses	0	0
Total
Fair Value	99	51
Unrealized Losses	1	1
Residential
Less than 12 months
Fair Value	0	3	2
Unrealized Losses	0	0	0
12 months or more
Fair Value	14	18	25
Unrealized Losses	1	3	4
Total
Fair Value	14	21	27
Unrealized Losses	1	3	4
Commercial
Less than 12 months
Fair Value		15	3
Unrealized Losses		0	0
12 months or more
Fair Value		8	14
Unrealized Losses		1	1
Total
Fair Value		23	17
Unrealized Losses		1	1
Large capitalization value
Less than 12 months
Fair Value	25	36	14
Unrealized Losses	2	5	1
12 months or more
Fair Value	10	6	12
Unrealized Losses	1	1	2
Total
Fair Value	35	42	26
Unrealized Losses	3	6	3
Smaller company growth
Less than 12 months
Fair Value		4
Unrealized Losses		1
12 months or more
Fair Value		0
Unrealized Losses		0
Total
Fair Value		4
Unrealized Losses		$ 1

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Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities which are categorized neither as held-to-maturity nor trading securities.

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Amount of the excess of amortized cost basis over fair value of securities in a loss position and categorized neither as held-to-maturity nor trading securities.

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for less than twelve months for securities categorized neither as held-to-maturity nor trading securities.

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The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

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