Fair value measurements - Changes in Level 3 Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2009
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amount of available-for-sale investment securities derecognized due to adoption of new accounting rules	$ 355