Significant Accounting Policies - Net Trading Revenue Including Dividend Income and Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounting Policies [Abstract]
Dividend Income	$ 45,910us-gaap_InvestmentIncomeDividend	$ 26,237us-gaap_InvestmentIncomeDividend	$ 2,253us-gaap_InvestmentIncomeDividend
Dividend Expense	$ (38,444)us-gaap_DividendsCash	$ (21,630)us-gaap_DividendsCash	$ (1,599)us-gaap_DividendsCash

Amount of paid and unpaid cash dividends declared for classes of stock, for example, but not limited to, common and preferred.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 405

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6957935&loc=d3e64057-112817

Name:	us-gaap_DividendsCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration