Variable Interest Entities - Summary of Changes in Liabilities of Consolidated VIEs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Variable Interest Entity [Line Items]
Net realized gains on debt	$ 91,000		$ 83,397
Changes in net unrealized losses from debt	(68,013)	(14,247)	(156,249)	(102,825)
Level III [Member]
Variable Interest Entity [Line Items]
Balance, Beginning of Period	11,347,332	3,700,536	11,834,955	3,189,837
Acquisition of VIEs		7,317,144		7,317,144
Borrowings		503,848	332,250	929,532
Repayments	(508,400)	(243,343)	(1,420,175)	(246,134)
Net realized gains on debt	(91,000)		(83,397)
Changes in net unrealized losses from debt	68,013	14,247	156,249	102,825
Elimination of debt attributable to consolidated VIEs	19,326	(59,772)	15,389	(60,544)
Balance, End of Period	10,835,271	11,232,660	10,835,271	11,232,660
Changes in net unrealized (gains) losses included in Net (Losses) Gains from Investment Activities of consolidated VIEs related to liabilities still held at reporting date	$ (17,662)	$ 8,456	$ 75,214	$ 93,021