Fair value measurements - Narrative (Detail) (Bank of America Corporation [Member], Cumulative Perpetual Preferred Stock And Warrant [Member], USD $) In Millions	Sep. 01, 2011
Bank of America Corporation [Member] | Cumulative Perpetual Preferred Stock And Warrant [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investment owned, total cost	$ 5,000