Significant Accounting Policies - Net Trading Revenue Including Dividend Income and Expense (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Dividend Income	$ 59,295	$ 63,971	$ 45,910
Dividend Expense	$ (47,436)	$ (42,398)	$ (38,444)

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 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 2: 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

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Balance Type:	debit
Period Type:	duration