Investments Other Than Securities - Summary of Investments Other than Securities (Parenthetical) (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
City of Chicago Taxicab Medallions [Member]
Investment Holdings [Line Items]
Gross unrealized appreciation	$ 5,846
Gross unrealized depreciation	0
Net unrealized appreciation	5,846
Aggregate Cost for federal income tax	1,392
City of Chicago Taxicab Medallions Handicap Accessible [Member]
Investment Holdings [Line Items]
Gross unrealized appreciation	172
Gross unrealized depreciation	0
Net unrealized appreciation	172
Aggregate Cost for federal income tax	$ 40

The aggregate gross unrealized depreciation for all Investments in which there is an excess of tax cost over value.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-12(8)(b))
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611133-123010

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-12C(11)(b))
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611225-123010

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 5
-Subparagraph (SX 210.12-13(7)(b))
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611282-123010

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Balance Type:	credit
Period Type:	instant