Fair Value Measurements - Summary of Gains and Losses Realized and Unrealized Relating to Level 3 Assets and Liabilities Included in Earnings (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date	$ 363,599	$ 162,300	$ 409,121	$ 425,901
Realized gains or (losses) and other settlements on credit derivative contracts [Member]
Gains or losses and other settlements on credit derivative contracts	2,944	3,829	9,271	13,376
Unrealized gains or (losses) on credit derivative contracts [Member]
Gains or losses and other settlements on credit derivative contracts	24,496	676	3,532	6,513
Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date	23,948	(10,425)	(4,192)	(9,889)
Interest Rate Swaps [Member]
Gains or losses and other settlements on credit derivative contracts	(1,156)	(295,523)	(46,191)	(361,955)
Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date	(1,156)	(257,764)	(47,419)	(327,371)
Variable Interest Entity [Member]
Gains or losses and other settlements on credit derivative contracts	341,578	438,466	462,009	750,933
Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date	341,578	438,518	462,490	749,878
Other Income [Member]
Gains or losses and other settlements on credit derivative contracts	(771)	(8,029)	98,952	13,283
Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date	(771)	(8,029)	(1,758)	13,283
Net investment income [Member]
Gains or losses included in earnings	$ (22)	$ (47)	$ 17	$ (3,419)