Derivative Financial Instruments - Fair Value of Derivative Instruments (Details) $ in Thousands	Dec. 31, 2015 USD ($) contract	Dec. 31, 2014 USD ($) contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$ 220,647	$ 169,423
Assets, Contracts \| contract	396,802	458,242
Liabilities, Fair Value	$ 194,723	$ 139,811
Liabilities, Contracts \| contract	430,121	469,292
Foreign currency, Futures contracts \| Receivable from/Payable to brokers, dealers and clearing organizations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$ 578	$ 1,212
Assets, Contracts \| contract	3,675	8,108
Liabilities, Fair Value	$ 955	$ 651
Liabilities, Contracts \| contract	6,586	9,090
Foreign currency, Forward contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$ 445	$ 60
Foreign currency, Forward contracts \| Financial instruments owned, at fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$ 445	$ 60
Assets, Contracts \| contract	1	1
Liabilities, Fair Value	$ 0	$ 0
Liabilities, Contracts \| contract	0	0
Equity, Futures contracts \| Receivable from/Payable to brokers, dealers and clearing organizations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$ 1,558	$ 1,790
Assets, Contracts \| contract	4,038	2,590
Liabilities, Fair Value	$ 1,743	$ 2,047
Liabilities, Contracts \| contract	3,432	3,085
Equity, Swap contracts \| Receivable from/Payable to brokers, dealers and clearing organizations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$ 0	$ 98
Assets, Contracts \| contract	0	1
Liabilities, Fair Value	$ 281	$ 13
Liabilities, Contracts \| contract	2	1
Equity, Listed options
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$ 178,360	$ 144,586
Liabilities, Fair Value	151,893	115,362
Equity, Listed options \| Financial instruments owned/sold, not yet purchased, at fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$ 178,360	$ 144,586
Assets, Contracts \| contract	360,469	426,747
Liabilities, Fair Value	$ 151,893	$ 115,362
Liabilities, Contracts \| contract	390,949	437,383
Fixed income, Futures contracts \| Receivable from/Payable to brokers, dealers and clearing organizations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$ 4,265	$ 6,432
Assets, Contracts \| contract	6,195	11,901
Liabilities, Fair Value	$ 4,037	$ 6,891
Liabilities, Contracts \| contract	4,891	10,628
Commodity, Futures contracts \| Receivable from/Payable to brokers, dealers and clearing organizations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$ 35,441	$ 15,245
Assets, Contracts \| contract	22,424	8,894
Liabilities, Fair Value	$ 35,814	$ 14,847
Liabilities, Contracts \| contract	24,261	9,105

The number of derivative instruments of a particular derivative asset or group of derivative assets held by the entity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5580258-113959

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Publisher FASB
-Name Accounting Standards Codification
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-Section 55
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-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226000-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=51814546&loc=SL51823488-111719

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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-Publisher FASB
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226000-175313

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- Definition

The number of derivative instruments of a particular derivative liability or group of derivative liabilities held by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5580258-113959

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-Publisher FASB
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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