Senior Debt - Summary of Debt (Parenthetical) (Detail) - Term Loan [Member] $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Debt Instrument, Redemption, through September 2017 [Member]
Debt Instrument [Line Items]
Outstanding borrowings, quarterly principal installments payable amount	$ 2.0
Term loans, year of maturity	2018-09
Debt Instrument, Redemption, through September 2020 [Member]
Debt Instrument [Line Items]
Outstanding borrowings, quarterly principal installments payable amount	$ 2.5
Term loans, year of maturity	2020-09

Amount of the required periodic payment applied to principal.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration