Variable interest entities and asset securitizations - Additional Information (Detail) (USD $)	12 Months Ended	24 Months Ended
	Dec. 31, 2012	Dec. 31, 2003	Dec. 31, 2011	Dec. 31, 2009
Accounting Policies [Line Items]
Securitization and transfer of residential mortgage loans to qualified special-purpose trusts		$ 1,900,000,000
Maximum Percentage of cash received on securitization of residential mortgage loans allocated to capitalized servicing rights	88.00%
Increase in loans receivable due to amended consolidation rules				424,000,000
Decrease in amortized cost of available-for-sale investment securities due to amended consolidation rules				360,000,000
Decrease in Fair Value of available-for-sale investment securities due to amended consolidation rules				355,000,000
Increase in borrowings due to amended consolidation rules				65,000,000
Carrying value of loans in the securitization trust	151,000,000		196,000,000
Combined outstanding principal amount of mortgage-backed securities issued by qualified special purpose trusts held by unrelated parties	23,000,000		30,000,000
Other assets for its "investment" in the common securities recognized by the company of various trusts	34,000,000
Total assets of real estate partnerships in which the company invested	1,500,000,000		1,400,000,000
Maximum exposure to loss of investments in real estate partnerships	270,000,000		271,000,000
Unfunded commitments included in company's maximum exposure to loss of investments in real estate partnerships	$ 71,000,000		$ 75,000,000