Variable Interest Entities - Carrying Amounts of Assets and Liabilities (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Variable Interest Entity [Line Items]
Investments	$ 2,134,061	$ 2,138,096
Variable Interest Entities [Member]
Variable Interest Entity [Line Items]
Cash	435,924	452,116
Investments	14,022,771	17,092,814
Receivables	520,841	915,161
Debt and other payables	1,402,393	1,752,294
Securities sold	45,824	32,702
Capital withdrawals payable	$ 99	$ 32,451