Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			3 Months Ended					3 Months Ended				3 Months Ended	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Asset-backed securities and utility obligations [Member]	Mar. 31, 2013 Other assets [Member]	Mar. 31, 2012 Other assets [Member]	Mar. 31, 2013 Sponsored Variable Interest Entities [Member] Transactions	Dec. 31, 2012 Sponsored Variable Interest Entities [Member]	Mar. 31, 2013 Financial guarantee [Member]	Mar. 31, 2012 Financial guarantee [Member]	Mar. 31, 2013 Variable interest entity [Member]	Dec. 31, 2012 Variable interest entity [Member]	Mar. 31, 2013 Consolidated entities [Member] Entity	Dec. 31, 2012 Consolidated entities [Member] Entity
Variable Interest Entities [Line Items]
Debt outstanding to third parties under secured borrowing transaction											$ 10,521	$ 14,588
Securities fair value	218,735		201,329								218,735	201,329
Fair value of special purpose entities	14,230		14,557
Change in fair value of special purpose entities					(327)	(756)
Number of individual transactions with special purpose entities							15
Number of transactions outstanding with special purpose entities							3
Total principal amount of debt outstanding							464,495	466,938
Weighted average life				8 years 8 months 12 days
Earned premiums for issuing the financial guarantee policies on the assets, MTNs and derivative contracts	30	145
Assets sold to special purpose entities	1,455	134
Claims on insurance policies issued to these entities	0
Earned fees for providing other services under its financial guarantee policies									2	11
Amount received under derivative contracts	93	138
Number of consolidated Variable Interest Entities													18	18
Consolidated VIE assets	16,751,505		17,841,863
Consolidated VIE liabilities	$ 16,548,127		$ 17,661,700
Average rating of assets held by sponsored special purpose entities	No longer than 90 days