Organization and Significant Accounting Policies - Narrative (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule Of Significant Accounting Policies [Line Items]
Cash and securities deposited with clearing organizations	$ 377.8	$ 424.6
Interest Rate Swap
Schedule Of Significant Accounting Policies [Line Items]
Cash on margin with counterparty to interest rate swaps	$ 87.9	$ 102.9

The amount of securities on deposit with clearing organizations and other entities which provide services necessary to operations of the Company. This item would include, for instance, securities advanced as refundable collateral for securities borrowed.

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 210.5-02.8,17)

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

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

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Paragraph 31

-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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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