Variable Interest Entities - Summary of Valuation of Consolidated VIEs in Fair Value Hierarchy Levels (Detail) (Consolidated Variable Interest Entities [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	$ 12,221,322	$ 12,689,535
Liabilities, at fair value	10,835,271	11,834,955
Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	60	168
Liabilities, at fair value
Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	10,463,405	11,045,902
Liabilities, at fair value
Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	1,757,857	1,643,465
Liabilities, at fair value	$ 10,835,271	$ 11,834,955