Variable interest entities and asset securitizations (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Variable interest entities and asset securitizations (Textual) [Abstract]
Carrying value of loans in the securitization trusts	$ 174,000,000	$ 196,000,000
Combined outstanding principal amount of mortgage backed securities issued by qualified special purpose trusts held by unrelated parties	26,000,000	30,000,000
Other assets for its "investment" in the common securities recognized by the company of various trusts	34,000,000
Maximum exposure to loss of investments in real estate partnerships	266,000,000	271,000,000
Unfunded commitments included in company's maximum exposure to loss of investments in real estate partnerships	73,000,000	75,000,000
Total assets of real estate partnerships in which the company invested	$ 1,400,000,000	$ 1,400,000,000