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v3.19.1

Preferred Units - Additional Information (Detail) - USD ($) | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Feb. 29, 2016 | Jul. 31, 2015 | May 31, 2015 | Sep. 30, 2014 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Class Of Stock [Line Items] |  |  |  |  |  |  |
Net proceeds from issue and sale of preferred units |  |  |  |  |  | $ 26,648,000 |
Distribution paid |  |  |  |  | $ 0 | $ 0 |
Preferred stock, shares issued |  |  |  |  | 0 | 0 |
Minimum |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Percentage of preferred unit holders |  |  |  |  | 70.00% |  |
Letter Agreement | The Gates Foundation | Series A Preferred Stock |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock, shares issued |  |  |  | 781,693 |  |  |
Preferred stock, shares issued | 361,884 |  | 218,646 | 201,163 |  |  |
Shares exchanged for preferred units |  | 419,809 |  |  |  |  |
Letter Agreement | The Gates Foundation | Contingently Redeemable Class A Preferred Units |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Preferred units classified as temporary equity |  |  |  |  |  |  | 781,693
Preferred units classified as temporary equity, cost |  |  |  |  |  |  | $ 5,000,000
Class A-3 Preferred Units |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Units issued and sold during the period | 2,005,348 |  |  |  |  |  |
Units sold and issued to investors, per share | $ 7.23 |  |  |  |  |  |
Contingently Redeemable Class A-3 Preferred Stock |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Units issued and sold during the period | 361,884 |  |  |  |  |  |
Units sold and issued to investors, per share | $ 7.23 |  |  |  |  |  |
Class A-3 Preferred Units and Contingently Redeemable Class A-3 Preferred Stock |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Net proceeds from issue and sale of preferred units | $ 17,100,000 |  |  |  |  |  |
Issuance costs | $ 0 |  |  |  |  |  |
Class B Preferred Units |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Units issued and sold during the period | 1,029,850 |  |  |  |  |  |
Units sold and issued to investors, per share | $ 13.53 |  |  |  |  |  |
Net proceeds from issue and sale of preferred units | $ 13,600,000 |  |  |  |  |  |
Issuance costs | $ 300,000 |  |  |  |  | $ 18,000 |

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