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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
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Earnings Per Share - Additional Information (Detail) - shares			3 Months Ended
	Apr. 11, 2018	Nov. 13, 2017	Mar. 31, 2018	Dec. 31, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Anti-dilutive common stock equivalents excluded from calculation of diluted earnings per share			1,101,513	1,549,440
Acquisition of ETFS Business [Member] \| Series A Non-Voting Convertible Preferred Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Business acquisition, number of shares to be issued		14,750
Common Stock [Member] \| Acquisition of ETFS Business [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Business acquisition, number of shares to be issued		15,250,000
Subsequent Event [Member] \| Acquisition of ETFS Business [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Business acquisition, number of shares to be issued	30,000,000
Subsequent Event [Member] \| Acquisition of ETFS Business [Member] \| Series A Non-Voting Convertible Preferred Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Business acquisition, number of shares to be issued	14,750
Subsequent Event [Member] \| Common Stock [Member] \| Acquisition of ETFS Business [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Business acquisition, number of shares to be issued	15,250,000