Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012 Margin Requirements	Jun. 30, 2012 Collateral Requirements	Aug. 28, 2008 Total Bond Market Fund Litigation
Commitments and Contingencies Disclosure [Line Items]
Aggregate face amount of letter of credit agreements	$ 350	$ 110
Alleged minimum percentage of fund assets invested in CMOs and mortgage-backed securities without obtaining shareholder vote			25.00%