Capital Stock and Derivative Instruments - Schedule of Vesting Shares Subject to Repurchase (Detail) - Restricted shares subject to repurchase [Member] - shares	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Amortization Expense [Line Items]
Unvested shares	4,133,020	4,053,640	4,039,591
Total unvested shares - subject to vesting schedule	577,464
Unvested shares - event dependent	3,555,556
1st quarter 2019 [Member]
Amortization Expense [Line Items]
Unvested shares	267,356
1st quarter 2020 [Member]
Amortization Expense [Line Items]
Unvested shares	210,612
1st quarter 2021 [Member]
Amortization Expense [Line Items]
Unvested shares	99,496