Segment Information (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	27 Months Ended
Sep. 30, 2012	Sep. 30, 2012 Syndicate 1200 [Member]	Dec. 31, 2011 Syndicate 1200 [Member]
Business Segment Information [Line Items]
Short-term investments	$ 121.1
Issuance of senior unsecured fixed rate notes	18.2
Assets associated with trade capital providers		$ 440.9	$ 548.2

Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph g

-Article 7

Name:	us-gaap_ShortTermInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant