Summary of significant accounting policies - Summary of Company's Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Interest Rate Cap \| Fair Value Measurements Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate caps	$ 0	$ 340
Interest Rate Cap \| Quoted prices in active markets markets (Level 1) \| Fair Value Measurements Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate caps	0	0
Interest Rate Cap \| Significant other observable inputs (Level 2) \| Fair Value Measurements Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate caps	0	340
Interest Rate Cap \| Significant unobservable inputs (Level 3) \| Fair Value Measurements Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate caps	0	0
Carrying value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	1,197,000	709,470
Estimated fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 1,188,985	$ 709,470

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13476-108611

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