Segment Information - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Face value amount	$ 75,000,000	$ 75,000,000
Securities borrowed	685,037,000	594,674,000
Securities loaned	$ 687,443,000	595,717,000
Short-Sale [Member]
Segment Reporting Information [Line Items]
Face value amount		$ 75,000,000
Coupon rate of treasury securities	4.625%	7.25%
Short-Sale [Member] \| Principal Investing [Member]
Segment Reporting Information [Line Items]
Face value amount	$ 75,000,000	$ 75,000,000
Coupon rate of treasury securities	7.25%	7.25%

Amount, after the effects of master netting arrangements, of securities borrowed from entities in exchange for collateral. Includes assets not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-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
-Topic 940
-SubTopic 405
-Section 55
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35710563&loc=d3e44140-110984

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=55761124&loc=SL6224234-111729

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

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