Financial Instruments - Schedule of Trading Securities Sold, Not Yet Purchased (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Securities Sold Not Yet Purchased At Fair Value [Line Items]
Trading securities sold, not yet purchased	$ 44,167	$ 99,613
U.S. government agency MBS [Member]
Securities Sold Not Yet Purchased At Fair Value [Line Items]
Trading securities sold, not yet purchased	13	16
U.S. Treasury securities [Member]
Securities Sold Not Yet Purchased At Fair Value [Line Items]
Trading securities sold, not yet purchased	19,722	11,755
Corporate bonds and redeemable preferred stock [Member]
Securities Sold Not Yet Purchased At Fair Value [Line Items]
Trading securities sold, not yet purchased	21,976	87,537
Foreign government bonds [Member]
Securities Sold Not Yet Purchased At Fair Value [Line Items]
Trading securities sold, not yet purchased		158
Municipal bonds [Member]
Securities Sold Not Yet Purchased At Fair Value [Line Items]
Trading securities sold, not yet purchased	128	143
Certificates of deposit [Member]
Securities Sold Not Yet Purchased At Fair Value [Line Items]
Trading securities sold, not yet purchased	2,325	4
Equity securities [Member]
Securities Sold Not Yet Purchased At Fair Value [Line Items]
Trading securities sold, not yet purchased	$ 3

Fair value of obligations incurred from short sales.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Paragraph 38

-IssueDate 2006-05-01

-Chapter 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Paragraph 19

-IssueDate 2006-05-01

-Chapter 7

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 940

-SubTopic 320

-Section 35

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6475908&loc=d3e42429-110968

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Paragraph 2

-IssueDate 2006-05-01

-Chapter 7

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Paragraph 40

-IssueDate 2006-05-01

-Chapter 7

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details